My influence 8 king didn't seem to help much when France and then HRE declared war on me (from being allies). Everyone else cancelled their alliance with me even though I was considered the largest military power and my king had more influence than theirs. Maybe that's part of the higher difficulty level?
Part of the "bloat effect" - when you're the largest, everyone else will need to stick together to protect themselves from you (and don't try pretending you won't be crushing them all in the next century or so )