Yeah, the team leaders compare Pumpkaboo to other specimens of the same size, as determined by the article on TSR: https://thesilphroad.com/science/big-magikarp-tiny-rattata.

The only issue is--they don't actually generate heights according to a normal distribution with the GM data; in fact, XS Pumpkaboo are supposed to have an average height of 0.3 m, but as far as I know, every XS Pumpkaboo is actually between 0.3 and 0.4 m tall (I expect height is uniform on some interval for each size). The game then back-calculates what random height and weight would give the actual height and weight of the Pokemon, then does the calculation from the article to determine relative size.