Resistance for different gear ratios Why are some gear ratios more difficult to pedal than others on a bicycle. 
It feels as though I’m missing some very simple piece of reasoning!
To add to Nathan's correct answer, the concept at work here is mechanical advantage. Different gear ratios offer different mechanical advantages, and let you divide or multiply your torque to suit different conditions.

It's the same with a car: you pull away from a stop in 1st gear in a car, not 4th, because the engine doesn't have enough torque to manage it in 4th.