>>29605320Difference in design aesthetic.
Pokemon focuses mostly on clear geometric shapes, so evolved forms will usually have a clearly defined, stylized look to them that you can easily track the features of.
Digimon are designed with a focus on intricate linework and details, so the bigger forms will naturally lend themselves towards designs that allow room for even more intricate, detailed linework.
Compare a Charmander and an Agumon sometime and the difference in what they're going for becomes clear. Charmander is a smooth, stylized collection of simple shapes, simplistic yet appealing. Agumon has a much more defined musculature, with eye ridges, visible biceps, all that stuff. They're fun to look at, but it's harder for the average person to remember what it looks like than most pokemon. Which is why Digimon is the eternal second banana to this franchise.
