YES, AND SHIVERING ISLES WAS MUCH BIGGER THAN DAWNGUARD IS.......WHY IS THAT SO HARD TO WRAP YOUR MIND AROUND?????
All SI has going for it is landmass. The Dungeons are about equal, maybe a slight towards SI. But not by much. Armor/weapon wise? I would give this is a tie. Spells? Dawnguard did add a few shouts, spells and 2 new perk trees to the few power SI added. Dawnguard wins here as well.
Also, Dawnguard did a good job integrating itself into Skyrim. Even the Soul Carin, a part of Oblivion, was integrated better than SI was. SI just felt like it's own theme, instead of an expansion; like two different games. Once you leave SI you lose your powers.
Also, there's crossbows that can be used outside of the DLC. Wild Werewolves. More creatures that integrate into the actual game, not just the DLC. Makes for a richer experience for the base game, not just the DLC.
So while SI has a bigger landmass. It was just it's own thing. Once you leave...you really can't carry your godly powers to Tamriel. Of course you kept the armor/weapons. But it never fully integrated itself like KoTN did.
Besides, one was $20 and the other was $30. Why are you complaining? Dawnguard did an excellent job adding to the base game, adding new features. Also it added weapons and armor; and new creatures. Two new story lines. New weapon type... Improved vampires. Anything less would have been a ripoff towards Bethesda.