Dicks and balls need a lot of blood, but having a lot of blood vessels in soft tissue hanging off your body is a huge weakness so ideally a male's package would be as small as possible while still being able to get the job done. This is the main reason most dicks change size.

Gorillas have tiny packages because the alpha male uses his physical strength to chase off any competitors. A gorilla's dong doesn't need to work any better then another male gorilla because they can just beat the fuck out of each other until one of them wins and then that lucky guy gets to bang all the females with his tiny dick.

In contrast, human dicks are evolved to compete against other dicks in impregnating the same woman. Apparently the weird shape of the head of human dicks enables them to essentially shovel other males sperm away from the cervix. Length obviously helps in making sure your load is blown as close to the cervix as possible. If humans evolved to have alpha males, these adaptations would be totally pointless.