Some people suggested that they will do a smaller DLC just to honor the Microsoft deal and then reserve the real big expansion for later and a simultaneous release. I don't think this exclusivity deal is paying well; lots of negative feedback, not to mention that you're going to loose the day 1 impulse buyers for PS3 and PC who will probably be more prone to wait for a 'gold' edition of Skyrim (especially with all the issues regarding random attacks and such) .
Oh I definitely agree that the next DLC is going to be smaller, I'm just a little baffled by people who keep saying that Beth said DG is going to be the biggest DLC Skyrim will ever see. I simply can't find a statement to that effect, and I'm beginning to wonder if the misinterpretation of that soundbite isn't at least partially to blame for both this myth, and the inflated expectations some people had for DG.

I don't buy that idea that Beth is going to intentionally hold back on the next DLC because of the exclusivity deal, though. Rather, I'd think that if they're doing a large DLC they want to give themselves as much time as possible to work on it, and probably end the wave of DLCs with a bang, so the next DLC will probably be smaller and more of a "filler".
And yeah, I'd be very interested in being a fly on the wall when they discuss the impact that the exclusivity deal may have on overall sales. I've heard a lot of PC and PS3 players say that even though it would have been a day 1 purchase, they're either going to wait for a GOTY edition or not buy it at all because it's not what they want. And while I'll buy it as soon as it's available, I can't blame PS3 and PC players who want to hold back until Beth or modders do something about the vampire attacks. I don't want them to at all personally, so I know that if there isn't a "completely get rid of vampire attacks" mod out there the first thing I'm going to do is go into the CK myself and see if I can at least temporarily neuter the bastards so they couldn't kill a Deer. And if they don't release within a week of the exclusivity deal ending(which seems like the most realistic possibility at this point), I'm wondering how many people will have just moved on. If it weren't for the fact that my internet situation will be iffy for a while in September and October, I'd probably forget about DG like I do with most DLCs thanks to GW2.
The problem, though, is that I doubt the lost purchases outweigh Microsofts..."donation". It really is a pretty anti-competitive business practice the more I think about it.