It's "only" 7 levels, but the zip is 16MB and it's nearly 50MB unpacked.
Eternal Doom is a 32-level megawad first released in 1996. Originally developed by a group known as Team Eternal, it was later acquired and worked on by TeamTNT. It features arguably the largest levels ever designed for vanilla Doom.
There have been three releases of the project, each one adding new levels. The third release, Eternal Doom III, also moved MAP15: Halls of Disarray to MAP32.
The soundtrack was done by Rich Nagel.