This is good advice and it was an obvious starting point to fixing the problem. The large size mixed with tiny boobs was causing my wife's nipples to get sucked into the pump (painful). The smaller size fits quite well.

Our problem is that on 2/3 units that we have, the seal between the silicone bag thingies and the plastic cap or flange is leaky. You can hear it weeping air with every pump stroke. I've checked for damage and I can't find any, but I know that it doesn't take much for sealing surfaces to become compromised. I guess my next move is to buy some replacement parts and see how that works. It's just frustrating that it couldn't work nicely out of the box for us.

Your good experience is encouraging though. Hopefully we can get them in good working order soon!