It's not about size of the change, it's about the complexity of the change. Increasing the brain size by four times is not a very complex change; some minor tweaks aren't either. edit: Also, the differences between chimp and human are pretty consistent with plain increase in the volume (and number of neurons), if you control for, hmm, cultural differences. You can teach chimps sign language; chimps can use and invent tools. We do it better, of course - larger brains are more powerful, perhaps the longer childhood also helps, etc.