First, let's start with the room count. Now keep in mind, the Deadly Levels (Level 7, Level 8, and a sub-level of Level
have some very complex rooms, especially on Level 8! So for example, Room #3 on Level 8 actually consists of Rooms #3A - #3G, so there are 7 fully developed rooms in that "complex". If you have the maps from the original boxed set, on Level 3, look at room #68 - it has 10 developed rooms in that cluster - #68A - #68J. The difference is that the "sub rooms" (#3B,#3C, #3D, etc) in Ruins of Undermountain III: The Deadly Levels are larger and more detailed and complex than anything seen before. Keeping that in mind, there are actually 231 developed Core Rooms and Areas of Interest in this release! 
There are 3 maps - Level 7 is a map the size of Level 1. The sub-level could almost be a core level in its own right - it's likewise a full-sized poster map much denser and more detailed than the Level 1 map. And Level 8 is a double-sized map, only instead of attaching north to south like Level 3, it attaches east to west, sideways, and it is packed with rooms!
Then there are the Dungeon Dressing Cards. Same amount as in the original box set.
We also have an Adventures Booklet as in the original, detailing 3 mini-adventures - two deep inside the dungeon and one that takes the PCs very far away, outside the dungeon, for a deadly adventure in a remote area of the FR!
We have 36 new magic items (almost every one cool and original, not boring "variants") including some new powerful tomes, a couple intelligent weapons, and even a new artifact!
Likewise for the 36 new monsters (Monstrous Compendium Sheets), 36 new spells, and a slew of NPCs (I believe there are 16 if I recall correctly).
So yes, it's a lot of material!
