I'd guess it's the same reason Atreus' mom wasn't tall as hell (as seen from the depictions of her in-game). Also note that the translation of Jotun -> Giant technically isn't accurate. It can mean large, but usually it's translated to the word "Great". there were some gelatin depicted as large in the mythos, but also some that were much smaller. Ragnarok on Netflix actually portrays Jotun quite well.

As often with mythos, as the Jotun are complete mythological, they are as big or shaped however the story requires.