I'm pretty impressed what you figured out by just experimenting. What you are describing is the transition between laminar flow and turbulent flow. The point w

I'm pretty impressed what you figured out by just experimenting. What you are describing is the transition between laminar flow and turbulent flow. The point where this transition occurs is governed by the "Reynolds Number". As you correctly discovered, the Reynolds number is influenced by scale and fluid properties (viscosity and density). The final missing piece is the speed of the fluid, which explains your observation: "Bigger works better, until suddenly it doesn't work at all."