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.
The amount of energy and power required to move your bicycle a certain distance at a certain speed should always be the same.
Power is torque times rotational speed. Shifting gears allows you to choose between pedalling harder (i.e. more torque) or quicker (i.e. more rotations per minute).
There is a very small amount of losses in the transmission (like 3%) which can vary depending on the gears you use (like ±1% or 2%). For example “cross chaining” (using the large chainring in the front and large sprocket in the back or vice versa) can increase losses. With internal gear hubs it’s even more pronounced. But you’ll probably be unable to feel (or even measure) it.