A wedding that is not small but not really big either.
And what makes it a big wedding? Is it the number of guests?
If so, where is the cut off between average size and large?

Alot. Somewhere in the ballpark of $35K.

A big vs small wedding is based on the guest count. Some people consider 50 or fewer to be small and anything more than 50 is huge. Others consider anything under 100 to be small and then you have folks who consider anything under 200 to be small. It depends on what is customary in your family and social circle.