The penis and erection process are not effected by the skin on the penis. Your penis itself is just as erect and just as long whether you can retract your foreksin or not. However, the APPREARANCE of your penis can be changed. If you have long foreskin which has no desire to auto-retract during erection, then the skn is not tight on the shaft, hoevere you still have the issue of proportions because of your hidden glans not standing on top of your penis and this may make the penis appear smaller than it really is. If your foreskin is short enough that it badly wants to auto-retracts, but phimosis prevents it, (which would cause discomfort during erection), you would then have 3 issues: The brain not commanding a full and complete erection due to discomfort, the "proportions" issue as above, as well as the base of penis being hidden by pelvic skin and/or scrotum skin that is pulled up by the tight skin on shaft. The last one is also an issue to those who are tightly circumcised.