It's one thing to invite a few people who are only casual acquaintances or distant family members of the bride and groom, but it's quite another to have a sea of unfamiliar faces at what is typically a fairly intimate event. Royals who marry don't usually have the luxury of low-key. By pure dint of who they are, VIPs the world over will want to be in attendance, and a great deal of them will make the invite list.
So if you hear hints from a bride you're friends with that she thinks her guest list is pitifully petite (especially when you know it's not!), ask her how she'd feel about a receiving line stretching for miles over the horizon. That is a serious surplus of hand-shaking, bowing, hugging and chitchat -- frequently with people who are practically strangers.