The size of the levels comes mainly from the data
for shading blocks. So even with a small WAD (the textures)
you will end if with a big level.
Try to use Flat shading, and invisible parts of block-surfaces.
I think that surfaces without shading, are then stored much smaller.

Also try to use WMB-entities, and reuse them in the level.
This way you lighning is not so nice, but the level-data is much smaller.