another way to look at it,
for a longer crank, your foot/leg has to travel a longer distance to complete one revolution in relation to the distance the bike will move forward. in other words a lower (easier) gear.

if you look at gear rollout chart you will see how far a bike will travel for one revolution of the pedals depending what gear it's in. now factor in the distance the foot will travel with a longer or shorter crank you can compare the distance the foot will have to travel versus the distance the bike will move.