Blight Town and Anor Londo are huge, for example.
I did quit precisely at Anor Londo.
The level is big, but empty. It has what, 20-30 enemies, 1-2 NPCs and a lot of running around. When you progress through the level it becomes even emptier. Kill the gargs and central tower has absolutely nothing. Open a shortcut wicked gate and you have no reason to get into the temple living rooms again. You pass through the painting cathedral maybe two times per game (during the first walkthrough and en route to DLC), but there's nothing to
do there. Also, living rooms, in my opinions, is one of the weakest parts of the game design-wise.
Same thing is true for Blight Town. You pass through it once to get all the items and that's it. There's no reason to return. For comparison, the Swamp is completely different - it has Quelana and that samurai guy, it's a crossroad of sorts and you can farm shards there. You have some reason to spend time there except being stuck in front of a new challenge.