How long your cranks are can alter the effective size of the gear you are using as well. Longer cranks simply mean that every stroke of the pedals makes you cover more distance and hence it could feel like you have a larger gear than you actually do. As an example, if you were to switch from 170mm cranks to 180mm cranks, it would be like you are increasing the gear size which would make pedaling at the same rate much harder. This is most evident when using a huge chainring like a 75-tooth one.