There isnt any way to compress all those stacks of ammo. I hope there will be, eventually. I am starting the believe - and I hope I'm wrong - that the only reason for the inventory bloat is to sell us more retainers. There is just no other logical reason why so many items stack to 12, nor why ammo still only stacks to 99 when crystals and shards stack to 9999.
Bring on the extra retainers! I have dollar bills burning holes in my pockets just waiting to be able to store more junk and market in multiple wards / cities :-D