The size of foreskin is like the size of every other body parts, it varies from one individual to another but if it covers less than half of the head and/or you've got a big head, then it can stay folded behind if you want, sometimes you don't have a choice and it just keeps on retracting behind the head so you just give up and your dick looks like it has been circumcised. If on top of that you're more a grower than a shower than your dick ends up looking circumcised even when you're hard.
I know many guys who have a short foreskin like that. One day, at the age of fourteen, fifteen or even later, the skin rolls back and no matter how hard you try to pull it back on the head, it just keeps on rolling back at every step you take. At some point you give up and put up with the friction of your clothes on the bare head until you get used to it (that's to say : lots of unwanted erections during one our two months).