I am not on a diet or trying to lose weight at the moment. When I talk about the baby weight it's that mum tum you can never shift, we're talking a few pounds, not stones ladies. The most recent conversation was because they were all tucking into Sunday dinner at about 2pm. As a child and still now I have never eaten a big meal at this time because of the times my parents are at work. So my body is naturally not hungry for a full roast dinner at 2pm. Partners grandmother asked me if I had eaten my dinner before I came as I had declined Sunday lunch and I said no. She then proceeded to tut, shake her head and give me a look as if I was trying to starve myself.
You don't have to justify yourself sweetie 
I also cannot eat shit loads in the day, and it's not unusual for me to have only had have a bowl of muesli and walnuts, and a cappuccino, 2 bananas, and a few grapes by 5pm. I just don't and can't eat a big cooked meal for lunch (or even a bunch of sarnies and some crisps and a kit kat like a lot of people I know do.) I do eat a decent meal at 5-6pm ish, and will have 2 or 3 chocolate digestives with a coffee at 8pm. But I just don't eat a lot at all.
I eat around 1600 calories a day, (400 below the recommended limit for women,) and I am still 10 and a half stone and a size 14, (and 5 foot 4.) I am perfectly healthy and am never ill. I have managed to get to nearly 50 without dying, so I must have done something right!
I agree that the OP's inlaws are not acting out of concern. They seem judgey, nosey, and intrusive, and they need to butt out.
@nutnerk
She tutted because you were rude. You declined the meal even though you hadn't eaten.
What is rude about not wanting to eat when you're not hungry?!
What a bizarre attitude.