Join Date: Jun 2010
Location: Northern Virginia
Smaller bags are a safety precaution--the bigger the bag, the more likely it is to rupture, and the more inertia the betta will get whenever the delivery truck makes a sharp turn or goes over a bump. The small bag helps the betta get knocked around a little less, and as has already been said, air is more important to a betta during shipping than a lot of water to swim in. If you ever get a betta shipped to you from a professional breeder, it will be in a small bag in a small amount of water.
Also, as far as the tap water thing goes--if you've seen the sink in the middle of the fish department, you'll notice that it has a pretty big cabinet underneath. In this cabinet is an R/O unit--not exactly tap water. It does not need to be dechlorinated, but it should be treated with a remineralizer.