I don't think he meant it to be the same size all the time, as in, erect all the time.
He's just saying that when he's soft it shrinks up (he's not a shower). And that from the shrunken stage, a woman (or man) looking at his penis would maybe assume it is small even when it grows.
I like the look of penis when it is nice and thick when it isn't hard. It isn't standing up straight, just laying there on his leg or whatever, but is still thick and several inches long. I think that's what he's saying he wishes he had.