To ask for pithy but classy comeback for SIL?

Look her dead in the eye and say 'You're obsessed with me aren't you, you can't stop thinking about me and my body. Others have said it too.' and walk away.

She "god, I'm just feeling such a heifer atm"
You " Well you know what heifers turn into - cows" then calmly walk away with a wistful smile on your face.

You know what, the classiest thing is to simply feel deeply sorry for her. She is so messed up, and that will have come from external influences, so what she says is always a reflection of just how desperately miserable she is on the inside. Don’t engage with that at all. ‘I’m really sorry you feel that way SiL, have you thought about therapy?’ If you want to be mean though. And if you want to be kind and add something positive into the world, say something kind, even when she is being mean. You’ll feel better for it. Basically just don’t allow any of the nonsense to wobble your head, your weight doesn’t define you or anyone else. It’s always easy to be mean, and makes you feel good for a moment, but if you can find the way to kindness it’s usually better all round.

If someone is pissing me off I tend only to respond to questions, comments just get a hard stare.
If someone says something very offensive I ask them to clarify or repeat it, people rarely do

@AmuseabooshI think the OP has said she’s put on some weight since her big loss so is anticipating a snide comment (am I right? You lost 11 stone but have put one and a half back on)?
Either way, I’d just act like I’m the best looking woman in the room. She says ‘I’m such a heifer but YOU look WELL’ ‘Oh thank you I‘m feeling pretty good these days’ big smile. Just take what she says at face value instead of thinking what she really means.

Reply:
Oh I've lost loads on the x diet. Would you like a link? Then proceed to talk her through a couple of recipes, maybe even getting other people to join in.. Come over here Ethel, yadda yadda etc

She'll often do that disengenous shit where she disparages herself (god, I'm just feeling such a heifer atm) when she knows that my comparative are much worse.
Answer 'Oh you don't look that bad.'

This is the best so far!

With anything where people try and make you feel insecure about something I try and be really annoying.
So if she starts on about it say something like 'I'm so sorry you feel uncomfortable in your own skin, have you ever considered counselling?"

“I didn’t think fat at all SIL. Maybe a bit tired though… is everything okay”?

'Oh, have a day off will you, Sharon? For once in your life?' and walk away.

I'd just give her a hard look, turn round and ask if anyone had seen a good film lately/football/art exhibition insert as appropriate.

Just roll your eyes and don't engage.

Engaging with her is encouraging a conversation, so if that's not what you want, grey rocking is the best way to go.

It might be a bit too much in this scenario but the best comeback I've ever heard is "unless I'm sat on you or you pay my food shopping bill my weight is none of your business"

SIL- Blimey, you’re looking slimmer these days.
You - Yep, I'll send you a newsletter so you can keep up to date. (with a smile)

SIL - You’ve definitely dropped some pounds since I last saw you.
You - Amazing, isn’t it? And I did it all without a live commentator.

SIL - Wow, you’re shrinking!
You - Yes, and soon I’ll vanish completely, which means you’ll need a new topic.

SIL - Wow, you’ve lost weight, I feel like a heifer next to you.
You - Careful, you’ll end up as my official weigh-in correspondent.

Laugh every time and then ignore her.

Well done for losing the weight OP! It’s never easy and you’ve done brilliantly.

I totally get the desire to have a good retort after years of her snide comments, but I think the classiest response is to just say “thank you / oh, thanks” and chat about something else, or to someone else. She wants to get a rise out of you so don’t give her the satisfaction.

The trouble with saying anything else is that she’ll turn it around and tell people you were horrible to her (or even fake some tears!), and make you out to be the snide one. People like your SIL are experts at this kind of crap. I had similar with mine and know how it feels. She obsessed over my weight and noticed if I put on or lost even a pound. (Thankfully she’s an ex-SIL now).

Yours might well be jealous now as well, so expect snide comments about WLI and it being cheating and all the other rubbish people say about every diet. This is a ‘her problem’ and definitely not you. Please just say “oh thanks” or similar and don’t take the bait.

It’s fun to imagine what you’d like to say though 😉

"It's not really polite to comment on people's weight, SIL. Let's talk about something else."

Comments like that should be met with silence.

She’ll want a response but don’t give her one. Silence will annoy her because she can’t gauge how the comment has affected you which is what her aim would be.

It’s really satisfying to be indifferent to someone like that and they pick up on your indifference. It gives you back power.

I'd look pointedly at my watch and say "well it took a bit longer than I thought before you mentioned weight but it's good to know you're still so predictable".

Maybe just politely offer her diet tips if she talks about being a heifer- “Sorry you’re worrying about your weight. I found cutting processed carbs made a big difference- have you tried that?”

‘I‘m such a heifer’

’Yes…it’s so easy to put it on isn’t it?’ sympathetic smile.

I'd just tell her to give it a rest and walk away.

No need for witty statements, she knows what she's doing.

Make a SIL bingo card with all her usual phrases on it and every time she says one pull it out in front of her and say "yeeeeesss almost a full house!"

God I'd have so much fun....

Yes! I think sometimes saying nothing and letting words hang in the air is the best response.

Oh my dear, the sooner you liberate yourself from this prison of weight analysis, the sooner you will have time and space to actually be happy. I want that for you.
very earnest and kind and a teeny bit condescending.