One has to also look at the species...monkeys are fast and sleek, penguins cannot run, plus...aren't baby penguins tiny compared to baby monkeys? As for Serperior...snakes do not usually give off the impression of being fat or bulky at all, hence Serperior's weight.
I just find it strange that Fennekin is a little heavier than Chespin though, since one would probably expect the latter to be heavier. Hey, maybe it's Fennekin's head for all we know, and can point to possibly high Sp.Def and Sp.Atk. XD As for bipedal or quadrupedal...I'd prefer Chespin to stay bipedal for a change of pace. Gives more possibilities for a wide movepool too. XD