I don't think we can make a 1:1 comparison in terms of raw content. They seem to have similar length in terms of gameplay hours for a single playthrough, true enough, however it's rather difficult to say it adds x hours when it comes to major additions such as Vampire Lord form, Werewolf perk trees, and Crossbows. NOTN simply added the new questline, armor, and a couple spells, no real new additions which changed gameplay. Think about it this way: KOTN was never a 'must-have' expansion, except for maybe the niche of paladin RPers. No one would say you're really missing out on much without it. However, if you're a werewolf, or a vampire, or an archer(to a lesser extent adittedly) it's difficult not to say that you're missing out if you don't buy DG. If you don't purchase KOTN, you miss out on a couple new items and a set of armor, and some quests. If you don't purchase DG, you miss out on the same plus a new form which introduces a whole different style of gameplay, a new perk tree for werewolves, and a new weapon type.
So yeah, DG isn't a DLC that you can simply say "it's the same length and amount of quests as ____, but costs $10 more! What a rip-off!" because it has some intangibles which seriously affect gameplay for the rest of the game.