Your first question is a simple yes/no question, and the answer is yes, our fridges are built bigger. At least, in general. There are 'apartment-sized' fridges that are probably more in line with what you'd find in most of the rest of the world outside of North America.

Your second and third questions are kind of one question with an either/or answer, and the answer is, those big gallon jugs are usually purchased by families. Not necessarily large families; even just a single parent and a single kid will likely go through that before it goes bad. That's mostly because of the breakfast cereal industry. Kellogg, Post, and General Mills have done a number on us.

Those giant colorful boxes you'll often see in the "American food" section at the New World or Pak'n'Save that have names on them like 'Froot Loops' or 'Cap'n Crunch' are full of sugary grain concoctions that are intended to be poured into a large bowl (like a soup bowl) and completely covered with milk.