Have you noticed that in all the eye witness drawings of space aliens they have no bulges to indicate any flaccid size dick at all. So in order for them to have big dicks they would have to be the ultimate grower. They may not even have dicks.
I've read before that the 3-4 ft aliens are actually androids. So they may be produced with no dicks as they have no need for such. The taller aliens, known as greys, are not androids but they don't have bulges either and therefore my guess is they either have no need for the tool, or have very small dicks to go along with their lack of emotion.
I don't know if we should refer to them in such a cold term manner: "space aliens", as there is chance they may have actually saved us by mixing their dna with earth cavemen in order to create modern man (the "missing link" then occurred). They are family who have a cold calculated way of going about things. They are that way because they eventully did away with useless emotions ages ago. It's our emotions mixed with power that may be this planet's undoing.
That's why some aliens are hanging around, for they know we are bound to destroy or almost destroy ourselves sooner or later. It's why aliens are abducting humans, so as to create a more sensible intelligent being on earth.