My (late) mother would do the 'oooooh I couldn't possibly manage all that'... She'd say that over a McDonalds hamburger fgs.
I don't know if it had always been there lurking in the background, as I never saw any sign of it growing up, she was always on at me about my weight but that was generally about sweets/sugar/desserts/snacks between meals - no issues ever over portion sizes at actual meal times.
However in her later years (bearing in mind, alcoholism won it's battle when she was just 63) she would cover up for the fact she felt sick all the time with 'ooh I couldn't possibly manage' and 'thats FAR too much food for ANYONE.. who could POSSIBLY eat all that' and I think such ideas had always been there.. but well hidden. Alcoholism and of course, making herself feel better and hide what was going on by making out that I'M the pig... brought it to the fore.
My father is... irritating, in that he will go on and on and on about how disgusting gross indulgent, piggish, revolting etc, something is (some sort of cake or dessert usually). But if you handed him a bowl of it or a slice of it, he will absolutely woof the lot.
His main aim appears to be to try to guilt whoever it is into handing over their pudding so he can eat it, but if you offer him some of his own he will refuse and say he doesn't want any. He only wants it if he has succeeded in making someone else feel bad, or taken something that was for someone else (he will also scarf down all of something from his fridge IF he suspects it was left there by mistake and wasn't actually meant for him).
No idea where that comes from, he was a toddler when rationing ended, his parents had plenty, his mother and his auntie who he spent a lot of time with as both child and adult, could eat for Britain, meals out with her took a long time as she wasn't fast but she could pack away a dinner. Carvery restaurants quaked in fear of her capacity. Then we'd be off back to hers and within half an hour the home made biscuits and cakes would come out (which Father never refused!)
Eat or don't eat it, eat what you like, I don't like thoughtless waste but I wouldn't want someone to stuff themselves to discomfort levels purely because its on their plate either.