In what way are you comparing them?
I thought I'd try replicating the shot in game:
From the same distance of 54,000 km on the unilluminated side of the belt at an angle of about 45°.
The resolution is about 134m per pixel - 2.5 times sharper than that Cassini photo.
The severe difference is obviously because the in-game belt is about 2km wide. Saturn's A ring, of which that photo shows a small part, is 14,600km wide.
If we got rid of the 2D belt 'sprite' and upped the rocks' draw distance to a few 100kkm, I doubt it would even be visible at 10kkm.
It's not my intention to criticise the engine, just point out the difference.
Looking forward to a Saturn-sized planet & rings in the future!