Hopefully this isnt a very dumb question, but I was just wondering why a 400 is a "big block" and a 403 isnt?
What separates a big block from a small block?

 

Basically the deck height. There are other differences, main bore, etc. The 403 was a max bore small block built as a bridge when the 455 was axed. Same deal with Chevy, they built a 396 big block and a 400 small block.