Titus: A large part of it is the facial expressions he gives him. Sonic looks like he just got stoned after receiving a lobotomy. The proportions of his facial features also seem off to me.
zestygod: avalon really does suck at this stuff. Plus he really seems to be into plowing children. He actually goes out of his way to make older characters seem underage, pretty gross.
I'm not totally against more realistic proprotions on Sonic characters, but Avalon is ass at it.