My issue is not about it being a cathedral. My issue is that with the first dungeon being so amazing and sprawling and epic, wouldn't it make smaller dungeons look lacking?

I think there's a nice juxtaposition between the larger more epic dungeons and the others that may be less epic but have very specific tones and themes associated with them. When you step inside one of the more epic dungeons, like the Tristram cathedral (and considering its past, shouldn't it be epic?) you immediately know you're somewhere important. Somewhere that looks and feels magical in its presentation and lighting, as opposed to, say... a cave. Still cool! Caves are still awesome, but you probably don't want magical purple and green lighting in a cave, it's probably going to have a much different and subdued feel. If it's a natural cave it may have light streaming in from cracks above, or if it's a mine it might have lighting from lanterns. Much earthier and natural. Then you walk into some ancient tomb of a powerful wizard, oh crap, this place clearly has something else going on. The lighting is a bit unnatural, maybe some sickly greenish hues to set a theme and mood.

That type of theming adds a lot to keeping the scenery changing and interesting. If you're just fighting demons against a grey or brown backdrop for hours and hours, days and days, maybe years and years... it gets boring. Interesting, themed, and contrasting scenery all help ensure visual longevity.

I think before too long we'll have shown a nice cross section of the dungeon types and looks. It's been too long staring at the one dungeon. Each one has a very unique look and feel, so it's really cool going in to each one and having a total change of scenery. It's all very exciting and we're all looking forward to sharing it with you and seeing the fan reactions just as soon as it's ready.