Mixture - both devices have 7.1.1 (11D201) as "Version".
Don. - All the photos are kept on my home pc. (M/S 7 Home pro) No photo streaming or any such. No replication with Apple cloud as I can't afford their fees. (And yes, they are all backed up on a hard drive and on BT cloud).
When I first sync'd with the ipad air I tried to get them all on. They all fit on to the ipad 2. That stuffed the air, with very serious indigestion, so I had to do a factory re-set. Apparently over-filling the air with data stops it from even syncing to reduce the amout of data......Since then I have only sync'd selected folders.
However, it doesn't change the fact that on the ipad 2 there looks to be some sort of undisclosed and non-alterable compression going on. This must be "close to the hardware" software or whatever it is called, as both have the same version and build of the software. I would very much like to be able to compress them in the same way on the ipad air.
As this is not a common issue in the apple communities, (and I've spent hours on there chatting, trying to find others with a similar issue) I've had chats with the Apple help desk. All chats were with very nice Irish people who seem to want to help. First lady gave up and passed me on to senior helper. He devoted 90 mins of his time (very generous) but could not find the issue, so he's gone off to consult with deeper techies.
I guess that there is something compressing them on the ipad2 that is not present on the ipad air. However, whatever it is, it is not in the Apple software that is released from time to time.