Aesthetic reasons I assume but plums can be bigger than apples. A lot depends on variety and even more depends on how they are grown. If you leave all the apples on a tree the apples can wind up very small even when ripe, less water also makes them smaller. I've seen ripe apples grape sized. Plums can get as large as average apples, when cared for similarly, minimal plums allowed per tree and fed and watered well, They can get large, especially if it's a plout (plum-outcross).