Accordingly, I've bluetacked the Aventine Royal elephant together, sans trunk and tusks. It is supposed to be rearing, but I've put all 4 feet on the ground for comparison purposes; it is the unpainted model at the front.
From the above view, you'll see that they are both pretty well the same size to the shoulder; roughly 53mm.
The build of the two elephants torsoes is not dissimilar. The legs and feet are similar sizes. The Empire body is rather longer, at 58mm to the Aventine's 49mm.
The big difference between the two is in the heads. The volume of the Empire head is perhaps twice that of the Aventine, largely because is is very much broader, with a bigger forehead and eyes set wider (perhaps rather too wide) apart. The elephant has a very large helmet and crest on the beast. I prefer the smaller Aventine head, but can live with the bigger head on the Empire model because it is such an imposing beast overall, with excellently depicted armour.
Broadly speaking, the two elephants would work well together. I'll be adding Aventine elephants to my Seleucid elephant corp as soon as they can sculpt them, and I can get around to painting them!