Like Bobby said, If you are overweight, the fat pad that rest just above your penis maybe really thick and covers it up.
Also, keep in mind that there are two classes of penises. The "Showers" and the "Growers". The Showers have a larger flaccid length, but when they become erect, they gain less length and simple become "rigid". Growers have short to almost non existent flaccid length, but spring fully loaded when erect.
And, to quote a HIGHLY abused line on this site, "It's not the size of the boat, It's the motion in the ocean."
If you have any SERIOUS concerns over it, talk to your doctor...and be honest.