They mention short cranks relative to the wheel size.
One of the first times I rode my Schlumpf in high gear I UPD‘d because I started to lean forward and when I was at a point where a quick correction was needed, I simply couldn‘t accelerate the wheel quickly enough to regain balance. Instead I got ejected (if I grabbed the seat at that moment things may would have evolved differently).
If you‘re riding a setup with a high [total gear ratio] (New concept: Total Gear Ratio - #23 by saskatchewanian) you have to correct sooner / ride smoother compared to riding a uni with a low total gear ratio.
When it comes to riding a small wheel, every bump and obstacle becomes harder to overcome. So you have to ride more attentively and anticipatorily.