Like the TARDIS, It's Bigger on the Inside

Here's what I want you to do: take your index and thumb, make a tight circle with them, and press it tight. Now try to fit a finger of your other hand through.
Do you feel how thick that ring is, trying to get through it? Do you feel how tight it is?
That is (at least!) how thick and how tight the muscle of the rectal sphincter is.

The human asshole does not just make up what you see on the outside. Now granted, men LOVE to watch an asshole clench and wink. They like imagining what things can slip through.

But it is important to remember that the rear gate does not only include what you can see with the naked eye. The ring of muscle is THICK, and it goes DEEPER. It is a very thick door with a very tight frame. It is a nice looking door, but a functional one, too.

Yes: Once a cock has passed that ring of muscle, the entry gets much smoother. A cock can slide, rather than push.
(Now I'm getting all quivery and wet)
But an average-sized cock will not actually be past that high-security gate until, oh, I'd say, the whole helmeted tip is in.

So if you get the first few millimeters of your cock in your bottom's
(or, if you want to make it more accurate, your bottom's bottom) (tee hee)
asshole, that is not the time to start thinking "Oh, YES! I'm IN! I ROCK!" Because you are not IN. You are still getting in. The door has been cracked open; you have not yet been beckoned inside. You still have to go slowly, carefully, and painstakingly
(get it? Pains-taking? Okay, I'll stop)
to make sure you don't go too hard too fast and get yourself thrown out the door completely.

Consider this your PSA of the day.