Are you synching all the photo folders on both devices or are you only selecting some of the folders on the ipad 2 so only some of the photos are being transferred?
Additionally, on the ipad air or you a member of some photo streams that the iPad 2 is not? Each of the stream photos would also be downloaded which could account for some size.
The photos on my iPad 2 take up half the size that they do on my PC so I guess there is some compression/ resizing taking place although I can't see any settings for it.