Why is it unfair to expect a sequel to have more content than its predecessors?
Skyrim does have more content than it's predecessors, Skyrim has unlimited content.
I think what you meant is, Why is it unfair to expect Skyrim to simply add on, instead of taking away, correct?
To that I say, because Skyrim isn't a traditional sequel. It's the same deal with System Shock and Bioshock, or Fallout 2 and Fallout 3. Want, is not expect. I want Morrowind II, I don't, and never will expect it.
Of course, some things in previous games are reasonable to expect. Decent writing for example. Because quality is a measurable constant, and after Fallout 3, we were lead to believe that Bethesda had become somewhat competent in the field. Only to find later in Skyrim, that we can't even refuse to sell Nocturnal our eternal soul (for absolutely nothing, mind you), or refuse to become a smelly dog simply to be a questing hero.
I'm not hating. I love Bethesda and the games they make, but I'm very jealous of the PC users who get an even better game than they started with.
Of course, i'd have the PC version if my Ancient Nord Computer didn't run like syrup going up a hill on a cold day...

I didn't mean to imply you were. Just a general observation of the attitude on these forums. The problem is, it's not even useful hate. They're usually passive aggressive "Ragebarks" like "If skyrim was an rpg itd be good".
My insane ravings are drifting the topic a little off course...
I just think a higher standard needs to be held by the term "Better", as provable quality.
For example, Morrowind plays like total crap, Skyrim plays infinitely better, both games are still far from perfect.
Skryim has a difficulty curve that plateaus around halfway through the reasonable content of the game. Morrowind did as well. Oblivion on the other hand, had a difficulty curve of near hyperbolic growth, since the Player was capped but the enemies essentially weren't. All 3 still haven't done it right.