1) kids taking the power seems to be the theme here.Originally Posted by DEX
2) Levi is Asian, he can't be big.
3) The kid is prepared to become a titan, and not for self or Marley Glory. That's the key thing here. You don't see me suggesting Gabi as the next Attack Titan candidate. Affinity and physical performance seems to have something to do with it as well.
As for the colossal, they never intended on anyone in particular to inherit it. The syringe was reserved to be used on whoever was in a critical condition. It doesn't HAVE to be a kid, but that's the way things are happening here. The only exception seems to be Daddy Jaeger.