It's a funny one, but: He was bullied when he was little. Tails' Adventure could be why that stopped. I do understand the confusion, though, the way it's written sometimes looks like it stopped when Sonic arrived, but I don't think he can suddenly grow from little to 'big' in the span of, what, a week I think I concluded as a minumum for that? Tails simply wanted to be "cool too!" when he met Sonic, nothing about him getting over or stopping any bullying.