It is rude when hosting other adults to comment on the portion size they like, to fat shame or to refuse more food if it is asked for and available. Quite simply it is rude to comment on what another adult wishes to eat. Quite why does another adult feel it is their place to make someone eat less?
I agree that in the past many families were hard up and food was short. When the food available was meagre, women almost always chose to go short themselves so the men in particular had a bigger portion feeling it was needed as they did hard manual work and because they went out to do paid work. Everyone was often a bit hungry in those days, but women often chose to sacrifice more rather than let children or men be hungry.
These days most people are not short of quantity of good, although some people have to be very careful still with their shopping. Not wasting food is a good principle but leaving people still hungry isn't a good idea. We should recognise that different people eat different amounts, even if it's quite different from what we would eat ourselves.
Clearly, OP eats with Mil frequently and has done so for years. However, if Op has never said the portion is too small, the MIL may well not know, especially if O has quietly eaten up her small dinner of years.
If mil has been asked for bigger portions and not to comment on baby bump size, then mil is being unreasonable to knowingly take it upon herself to force Op but eat less. However, is this the case or doesn't she realise? Lots of older ladies et like birds and are surprised how much some women at even men set ......but if you don't say, you can't expect them to know.
Fine if you go very occsicialloy to someone's for dinner and you get a very small meal. Bit disappointing but not a big deal. But if yet eat with someone weekly, why not be clearer about it it? I suspect Op has a bit of an issue about food - perhaps feels she is fat shamed or people think she's greedy or is unable to ask for more because of her issues with food, and this makes her hyper sensitive about it all. Otherwise it's really not a big deal to quite simply ask for a bit more. I'd eaten tith inlaws on multiple occasions - sometimes I'll say to MIL 'oh please give me a smaller portion' and other times 'oh please could I have a bit more of X' without giving it some thought. I can do it because I don't have food issues and I don't feel judged in my eating habits. Lots of people to have food issues and I gained they are fat shamed when they aren't to simply fave people with different appetites tp the,selves. Being open about it is is the best if you possibly can be honestt when you'll et there for years to come, best not to let this grown into a big issue. If you do have food issues, then that needs sorting out.