General

in the near future could fat become fashionable? might it become desirable for the general population?

Being fat went in and out of fashion multiple times during human history.

As social acceptance for previously taboo things seems to become more common, I would guess being fat can become more accepted, even fashionable, even if not generally, but at least for a larger and more visible part of society. As society is becoming less monolithic, there will probably not be one single beauty standard for everyone, and being fat might be among one of the most widespread beauty standards.

However, I suspect this is only for those who still fit inside the BMI chart. Being barely able to walk, belly-to-the knees size would almost certainly never become a fashion in real life, and will remain a niché.

As we know people have gotten progressively bigger (height and weight wise). I'm kind of curios how big the fat people of the middle ages actually were.

Not as long as the diet industry makes billions of dollars off of fat people hating themselves.

In space and on the moon the human body, may well just get fat in the future as the human population's metabolism will still be that of Earther's and will store unused energy as fat... so weightless fat people would become a norm but also limit them to low gravity planets or space alone! LOL