Push Cubes. Blow Stuff Up.
My understanding is that your hand size is sort of your stamina. Thematically it makes sense that the Spellweaver has a smaller hand size than the Cragheart.
It could also be balancing since the Spellweaver is likely more of a glass cannon archetype than the tank.
Granted, I could be wrong. I have not read the current draft of the rules and when I did read a draft it was long ago.
This is how I interpreted it, too.