Is it just me who finds this odd?

I think you're over thinking this.
A hangover from Nineties diet culture when we were obsessed with women's weight/bodies?

These days luckily there's much less body shaming and much less open discussion/comment about women's bodies.

I wouldn't mention weight loss/gain to anyone unless I was invited to. I wouldn't assume it's passive aggressive from your family member.

Regarding the leaving of snacks/treats etc. Keep them for your guests or bin them.

You know them.

If you think they're giving you unhealthy stuff deliberately then they probably are.

I've lost 14 stone over the last 6 years. You know instinctively from peoples mannerisms who is offering you cake because they've always offered you cake and they don't want to offend and who suddenly starts pushing more and more unhealthy stuff because they don't like you leaving your fat pedestal.

I've stopped meeting my cousin for lunches over the last year as the more weight I lost (and I needed to lose a LOT!) the more she wanted us to be going to cake places and wanted me to eat some of her 'treats' constantly. When I pointed it out to her she declared that "fatties are fat, even when they go slimmer for a while" as if she was doing me a favour in bursting by weight loss bubble sooner.

I never comment on size or weight. I might say ‘you are looking well’ or ‘you look lovely’ if someone is glowing or has dressed up, but never about size or shape. I think a ‘well done’ for losing weight, implies you were somehow not doing well before. For some people a loss of weight is to do with ill health or stress. I try to avoid any comments that associate worth with weight. It’s not helpful. Size doesn’t indicate health. Some very thin might be drinking alcohol to excess and smoking like a chimney. Someone larger might eat healthily but too much but exercise a lot and have a healthier heart than someone naturally thinner.

I think it’s great that you are looking after your body. It means you’ll be able to enjoy more things for longer. If you need praise then praise yourself. What anyone else thinks is inconsequential. And if you really want them to say, then tell them what you’ve done.

To be honest I'm slim and always have been and my family always try to hand me left overs when leave. Sometimes I'll take them, other times I'll say no thanks.

I wouldn't mention your change in lifestyle either unless you specifically brought it up. Really it only goes down well if both parties are aligned in their ideas and motivations, and are on the same journey, otherwise one side tends to feel worse after the conversation because in general it makes you reflect upon your own attitudes.

No I wouldn't consider it strange. I don't comment on anyone's weight, definitely not my family's, and I offer treats to everyone because I think most people eat delicious things sometimes, even skinny ones!

I might fond it odd that close family hadn't noticed (or hadn't mentioned) that I had lost a large amount of weight and looked very different. I wouldn't expect them to stop offering me 'unhealthy' food though. They're just carrying on as normal because you haven't asked them not to.

I don't think we should be commenting on other people's bodies.
Bring it up if you want to talk about it. But also I think someone being neutral about it is perfectly fine.
I have lost a lot of weight and find it very uncomfortable if it's commented on.

What was the previous dynamic with that person in terms of offering compliments, offering food and do they tend to be judgy of people they see as "vain"? And are they overweight themselves? All those things might be relevant. (I'm assuming your weight loss is obvious and couldn't have gone unnoticed.)

My mum has a weird attitude to complimenting people, particularly her own daughters. She'll tell you if she thinks you're looking chubby or too scrawny but if you actually look great/ just right she won't say a word. She did say once that she never gives compliments because it makes people vain and bigheaded.

Offering food, may just be habit if they've always done that. Maybe jealousy or self judgement (i.e. need to stop OP losing more weight or I'll look even bigger next to her).

Maybe someone who sees appearance as unimportant and doesn't pay it any attention even if noticed.

some people prefer you stay overweight they get used to you being the chubby/fat friend/relative/colleague

and will happy for you to return to being overweight

In that case, don't mind their feelings more than they care for yours.

Say no thank you to the food offerings. They obviously couldn't care less about you making better choices.

Well done on the lifestyle change. It's no mean feat 👏🏾.

If OP has lost a significant amount of weight, I'd say it's a given she will have eliminated/dramatically reduced consuming all those things...

Reading between the lines, I'd wager that they're still offering you unhealthy food because they're jealous of your significant weight loss.

Next time, politely but confidentially say: 'No thanks, I don't want to undo all my hard work eating cake!'

Tbh you could be really ill and that’s why you’ve lost weight. One of my colleagues lost a lot of weight and someone was like “what’s your secret” and she just quipped “cancer”. She had quite a sharp sense of humour and honestly didn’t mind but you can’t expect people to be mind readers. If you refuse to address it with them then they might feel that they shouldn’t be the one bringing it up.

Then don’t and just chuck it in the bin as soon as you get home. Or invent a food allergy or intolerance which means you can’t eat that food.

Why would they be offended tho at you saying you're trying to maintain your weight loss?

Maybe they object to the way you've lost the weight.

Maybe you're one of those persons for whom weightloss makes you look haggard and sunken, and it would be unkind to bring that up, so best avoided.

Couldn't be any number of things really.

It is very clear to me that one of my family members has lost huge amounts of weight using injections. I have personal objections to the way these are bought, unscreened, lied about to gain access to them, and caused a dramatic shortage for diabetics as they were first popularised. I therefore will not comment or make reference to her weight loss.
Could it be similar? Or just plain old jealousy?

Unless someone told me they'd been diagnosed celiac or gone veggie/vegan then I'd still supply the usual treats for my friends, regardless of their size. You should just bring some healthier stuff next time and just say you're trying to cut out xyz, would you like to try this that I made etc.

Some people don't comment on others appearances at all. They just don't. Some always do. 'oh, you look nice, you've lost weight, I love your shoes etc' neither one of those options is perfect. Just enjoy their company. If you want to talk about your weight loss then just raise the subject in conversation.

No, I lost 3.5 stone and no one in the family really noticed or said anything untill I was 2 stone down . Once I got to 3 stone suddenly everyone noticed.

I think it like not noticing children growing. It's only when you haven't seen them for ages you realise.

The issue with commenting - ‘you look great’ when someone has lost weight, is the inbuilt implication that they have been scrutinising your appearance and until now, thought you looked less great. It’s a tough one to navigate.

Some people are feeders. My mother was one. The emotional blackmail even when the doctor had told me to lose weight was unbelievable. The pouting, the sighs, the persistence . . .