I’m not sure how well I can explain this. But think of a seesaw. Depending on where the fulcrum is, more or less movement (or pressure) will be required at one end in relation to the other. If you have a 50 lb. child at each end, the fulcrum should be at the exact center. If your two participants are of unequal weight, then you adjust the fulcrum so that more weight (and movement) is required at one end and less at the other. The key on the melodica is a lever, and the distance you press the key is greater (and the pressure required is less) at the front end than at the back end just over where the key pad opens. So the longer the key, all other things being equal, the finer control you will have over the movement of the key pad and the amount of air released. Consequently, all other things being equal (which of course they usually aren’t), the longer the key, the finer control you will have over the amount of air. In theory at least, it should be easier to bend at the front end of the key and get harder as you slide up toward the back, and it will be impossible to depress the key when you reach the fulcrum. I suspect that even just thinking of the key as a lever makes it easier.