Yeah I was also surprised at how large it was when I first got it. But I think that was because it was copper. The next jump up, to 5 sen, switched to silver and shrink the coin to the same size as a dime.
Yep, In the last photo above, the top right silver one is the 5 sen. 90% silver, size of a dime.
EDIT: Oh my bad, that's the 10 sen. Hmm... where is my 5. Hold on, let me look in my silver box.
EDIT, pt 2: ok, I was confused. The 5 sen was also copper. I actually can't find mine. I'll have to go buy some more. The 10 sen is where they switched to silver and it's about the size of the dime.
I think those older Japanese coins are so cool looking. The designs are so neat.