Would a 300/2.8 (or anything else for that matter) be any smaller if it was made for a crop camera?
300/2.8 is 300/2.8. Unlike smaller cameras, we don't usually talk FF-equivalent with crop DSLRs so the lenses are the same.
I'm very happy with a 70-200/2.8 and 300/4 on my 7D. I like the focal lengths and I like the apertures. These would be more or less the same size if they were redesigned for crop, wouldn't they?
A 300 on a crop sensor could of course be smaller, and really smaller on a short back plane mirrorless - e.g. Olympus 75-300 is tiny (relatively) - just compare to Canon 70-300.
How can the front element of a true 300/2.8 or 300/4 be any smaller than they are now? I thought the focal length and aperture of a telephoto lens pretty much determined that.
The Canon 70-300 4-5.6 is 3.0" D and 5.6" L.
The Olympus 75-300 4.6-6.7 is 2.7" D and 4.56" L.
So yes, the Olympus is smaller, but only about an inch shorter. Hardly "tiny" relative to the Canon. The Canon is also a faster lens (although they both use the same size filter, 58mm). The Canon also needs to have the IS mechanism inside (but that's a different discussion).
Based on size, I don't see a huge benefit from one to the other.