If you're big as in thick, initial penetration is quite tricky. If not done right it can be very painful, and even with care it can be uncomfortable. I think most men think get hard, arouse your partner, all's good. But when you're attached to a thick penis, this doesn't work. You have to be very considerate to your partner's initial reaction, monitoring her body language to make this difficult task as pleasant as possible for her. Start slow, penetrate steadily, read her body, stop if necessary.
So this is something that well-hung men need to learn and should learn.
For me, my partner can't be on top to start, so we generally start in the missionary position, even if briefly. Woman on top is tighter, and tighter means difficulties. If I penetrate her with me on top and she's taken it all, then we can change positions. Another trick big men should know about.
And then sex can be routine, repetitive, too fast and so on, which is what Hayley mentioned. I think for bigger men, everything tighter and more intense, so it could be more difficult for us to last a decent time, and we may need to work harder at this than average-sized men.