The expansions were never as long as the original games. Reign of Chaos is much more longer than Frozen Throne, original Starcraft is also way more longer than Brood War, same for C&C series. DLC is something that you play for couple of hours, they were never suppost to be lengthy. Lair of the Shadow Broker is perfect example how DLC is done good.
The last good expansions i've played were DA: Awakening and Heart of the Swarm and i can tell you even on my first playthrough they were much shorter than the original games which is to be expected. However saying 10-20 hrs content = little DLC? Care to tell me which DLC did that? The expansions barely lasted that much let alone DLCs.
This is how they start out though. This does not seem so bad, so they do it. But then the next little step up to using DLC does not seem so bad, so they do it. And they keep going up and up and up, because each little step does not seem so bad, until they are all the way up ♥♥♥♥ creek.
Believe me, this is 100% how all other companies started out... They saw the money gains, and before we knew it they were selling skins for $5 each.