Twunk is just a hunk but young no ?

My understanding of Twunk is that it's twink that is either muscular, like still small/skinny but muscular. Gymnasts would be twunks. I also personally would think guys who are "skinny fat" are in the twunk category. Just any guy that is small frames but has more muscle or more fat on him than what is generally on a typical twink.

And no need for euphemism with the last one, just “fat” would do.

You're thinking of cubs/bears for the bigger guys. Stocky/beefy is like, rugby player type builds, where they're muscular but with a good layer of fat over the muscle too. Can't really call them fat but you can't really say they're ripped either. Hence, stocky/beefy.