One thing is 'twichiness' due to geometry, wheel size and contact patch. Another thing, quite different, is 'wobble'. A bike, any bike regardless of size, wobbles when its wheels are out of true or bent, its drop-outs are not precisely parallel, the wheel axle is not seated evenly on the drop-outs or the frame is bent. Wobble suggests poor quality, a need for maintanance or damage. The 'wobble' that people experience are the first attemtps at and unfamiliarity with the lively steering of small wheel bikes. They try them once, find them unfamiliar, never ride them long enough to master the steering, and therefore proclaim them 'wobbly'.