What are you storing your BM in?
Also, if you start pumping more than 6oz from one side, you can get the 8-9oz bottles from Gerber and the threads fit on the horns for the medela pumps. I know it sounds like a lot, but I was actually getting up to 6oz on the left and 8oz on the right in one sitting when I was EPing for the twins. Those little 6oz bottles that come w/ the pump just weren't doing it. Plus, those colored Gerber bottles are super cheap, like $6 for 3 of them.