Partially empty food containers, does the scam really work?

Buy Blue Bell ice cream. http://bluebell.com/the_little_creamery/still_a_half_gallon.html

Aren’t the bags filled with nitrogen to prevent spoilage also?

I just came by to post that. Confirmed at the store, still sold in half-gallon containers.

Yes.

I don’t know about the US but over here, ice cream is sold by volume, not weight, so it pays to make the ice cream as airy as possible, for which we can genuinely blame Mrs Thatcher as she invented the process when she was a chemist.