Size, weight, and power may matter in tumbling eventually, but not at the RO-BHS-BHS or BT stage. At that level it is all about technique. Another thing is that if a kid has fear issues or takes a long time to get back tumbling skills, sometimes it takes time to get comfortable with going "all out" and actually using their power in tumbling. I'll bet the coaches are doing drills to help them connect the (pretty dang complicated) parts of tumbling all together and it will come in time.