That has a lot less to do with the size of your cock and way more to do with the size of your ass.

Lose some weight. I'm well endowed and have only ever had an issue of "where penis go" when I tried to go while fully erect when I was a string bean of a teenager, and again as an adult when I weighed around 275 lbs and tried to go with a chub.

Might also be a grower vs shower thing too, but your experience is not anywhere as common as you think.

Edit: Would be much more of a problem if you're using one of the round bowl toilets instead of the "long" ones with an oval shaped bowl, but the only non-long ones I've ever encountered were in places that hadn't replaced theirs since around 1950.

But then, wouldn't it flop tothe floor? It makes no sense!