I’ve been to many of the “special” circumstances weddings you listed, but have been to MANY more where the hall was filled with 2nd cousins, just because people feel lame making a wedding for “only” 200-300 people.
My barometer? The “contact list” in my phone / e-mail. If I’m not staying in touch with you on a regular basis, I’m not sending (read: obligating) you an invitation.
And believe me, no one stays in touch with 600 people. Yet I know people that mail out 700+ invitations. What for?