I have to admit, the only time I'll get hit by an AOE is if I'm in or on the edge of the red marker and that's exactly how it's supposed to be. I've only got up to Ifrit, so I can't say for any higher level stuff, but I'm sure I read in the beta patch notes that the red marker is an exact area of where the damage will be, so if you're still being hit outside of the marker, I'm thinking either lag or a glitch.
