That would take way too many resources to implement, which are simply not there as long as HTC is busy building AH3. "Yet smaller maps" (I read that as: even smaller than the ones we have today) would have to be made first, to MA standards.
Any kind of system that involves a map switch when numbers drop, is going to cripple action even more, as it involves an arena reset. We had such a thing once, remember? With less players than back then, it would take far longer for the arena to get back up to any kind of numbers.
Furthermore, I don't see the need to keep large map up at all, at this moment. For about half of the day AH has below 100 players during the week, with barely going over 200 for a limited time during US prime. It's not that a small map would be that crowded. After the big arena split, we played with substantially more players on those small maps.
Don't get me wrong, I love the many options the large maps can give us. I was very much lobbying to get them back to us after the Arena split. Compello is my favourite map of them all.
But the time for them has gone. One can only hope it comes back. But until then...