I'm learning the tricks of this bent stuff.  one observation is a weave I get with the whole bike at a higher speed. It is not a front wheel shake.  It  doesnt alarm me but i'm wondering if the smaller wheel sizes have this "problem".  

There's also just a basic difference in the physics of balance...  think of a metronome -- when the weight's up high, it's a slow oscillation -- when it's lower, it's a faster oscillation...   But Greg's right, the combination of pedal steer and non-relatxed grip also contribute.  Try "laying" your hands on the grips, keeping one or two fingers on the brake levers at all times, which will help you relax.