I would prefer to keep the citizens and then have them starve down after resistance ends than to have them instantly lost when I capture the city.
In the former case, I could rapidly build more farms, repair pillaged farms, buy more tiles, reallocate tiles between cities, buy food producing buildings, buy MCS allies, etc. to prevent starvation.
In the latter I could do nothing.
I don't like the idea of a variable citizen loss from city capture depending on some value of "how many citizens the city can support" that may not be very close to the actual numbers of citizens that I could have it support when I got to micromanage.