As far as the plumbing goes, it won’t be that different in the rectum/colon area, other than proportional size which would apply anyway whether it’s a tall or short male or female.
Guys, of course, have the bonus of the prostate getting stimulated. But for the dick giver, it likely won’t be different physically.
Everybody’s built slightly different, of course. The depiction of that area you see in medical diagrams, such as the one I used above, are the “idealized” version but actual person-to-person build can vary wildly. To the point where new medical students have trouble identifying what parts they’re looking at because it just doesn’t look like what they’re told it will.
Some examples of potential deviations in that anatomy:
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Now to your situation...
If he tries to push his entire dick in “to the balls” every time and this is hitting a painful wall for you, then you both may just have to accept that he can only go ¾ of the way in and be OK with that.
“Oooo, babe, you’re
so big!!” might help his ego there.
With that limit in mind, he can still fuck the bejeesus out of you and cum a mighty cum in your ass (with condom, of course). At least until you’ve “toyed” with your hole on your own and worked the kink out. Not everyone is a natural Armond Rizzo, after all.