When you hold a wedding, it’s likely to be influenced by weddings you’ve attended in the past. Mostly this has obvious boring consequences; nice features of weddings you’ve been to (open bar) might be copied, horrid features avoided. Slightly more subtle, but still unsurprising, is something like anchoring bias. The weddings you have attended in the past define a “typical” wedding, and good or bad, any new ceremony is likely to be heavily based on that model.

In summary: good features propagate, bad features die out, and culture persists.

One feature of a wedding, though, is a little different: its size. More people attend larger weddings. If, on a given day, ten weddings are held, of which half have ten guests and half have two hundred, then twenty times as many people will have attended a large two-hundred-person wedding, even though only half the weddings that day were so large.

This has consequences for the idea of a “typical” wedding. Former guests, when planning their own ceremonies, will look to the set of weddings they’ve attended to determine what is typical. But that set is inherently biased towards larger weddings. As a result, when guests try to design their own weddings based on what is typical, there’s pressure for the size of weddings to grow over time. Of course, this growth is not without bound, because there are practical reasons why a wedding can be too large. But we’d expect more weddings to “err” on the side of being too large than too small.

After a quick rephrasing, this is the same as the friendship paradox. Most weddings you’ve attended have had more guests than yours had (or will).

Finally, note that there are other reasons why weddings might grow (a large wedding is a nice signal of wealth/status, for instance), but those are all dependent on external societal factors. The evolutionary pressure on size is independent of culture, laws, or customs.