Are you asking about infantry or cavalry?
I would say the GB foot are perhaps slightly taller than Aventine, but of similar bulk. If this appears to be a problem you could add a thin spacer to increase the height of the Aventine.
I don't have any GB Sassanid heavy cavalry left and I have never seen the new GB Sassanid HC horses. I do have some LC on Aventine horses. The GB cavalryman might be slightly bulkier than the Aventine, but not by much (although some of this impression of GB bulk may be because shields & archery equipment are all molded on to the GB cavalryman).