There are two kinds of penises: 1) a grower, which expands and lengthens when erect and 2) a shower, which looks big most of the time, but doesn't get much bigger after an erection. Approximately 79% of men have growers and 21% have showers. In other words, a man whose has a small flaccid penis might have a surprisingly big erection. Conversely, a man with a big, limp penis might not get a significantly larger erection.