Super Power Countries Argue My Monster Bigger Than Your Monster

Thought I'd go with a Mine is Bigger than Yours theme. Here we have America and Japan represented by their iconic monsters. America which is geographically a much larger country seems to be represented by a much smaller monster. On the other hand, The smaller country, Japan has a ridiculously enormous creature that seems to dwarf anything that the U.S. has to offer. So, what is really going on here?