I'm sorry but did you even play Shivering Isles? Cause I seriously don't think you could say that if you did. Yes, we all know that SI added a bigger landmass, however not once have I said that that is the reason why it had more content. Like I've written maybe 5 times now, there were more dungeons, more weapons, more armor, villages, a city....literally the list goes on.
There was not more dungeons!!!!! There was not more Armour !!!! There's 8 more spells!!! There's a ton of More random encounters (oblivion didn't even have them) 2 more skill trees!!!! Crossbows!!! Around the same amount of side quests, but even more if you count the radiant quests!!! There was WAY more creatures added in dawnguard!!!! TWO unique faction main quests, Two new bases, two new large areas, new mounts, MY list goes on AND ON!!!!