I can't say what's in the heads of manufacturers but if the same manufacturer uses smaller wheels on long cage RDs and larger wheels on short cage it might be to compensate for using the same body on both versions. The Jockey wheel diameter factors into the cassette clearance, and changing wheels is simpler and cheaper than building two complete models to accommodate various cassette sizes.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.