No, actually. Pokémon of every species exhibit a certain amount of physical variance but by and large they adhere very closely to a breed standard, with a few notable exceptions. Pumpkaboo and their wildly varying sizes are one such exception. Samhain here is the smallest I've ever seen personally, but the smallest on record actually has him beaten out by a centimeter's worth of diameter or so.

[He does pronounce Samhain correctly. Of course he does.]

... though now this has me wondering if Cofagrigus vary in size depending on the stature of their living counterparts. I ought to ask Mallory how tall she used to be.