[Anthony]

While a larger creature can fit more brain power, there's no strong reason to think it will actually do so, and in any case since you're converting magical beasts mundane scaling laws are of limited relevance.

On the issue of DX, a large creature will have larger manipulators, and will thus have issues with manipulating small objects. It will also take somewhat longer to perform any given action. It's not clear that this is best implemented as DX, though.