Actually - I intended to write a tutorial about this, but never got around to it completely.
A .map filled with cubes, is 79kb, whilst the rmf is 317.
A .map filled with func_walled cubes is 86kb, whislt the rmf is 329 kb.
A .map filled with cylinders is 131kb, and the rmf is 528 kb.
Now:
A .map filled with clipped cylinders, all over, at random, is 566 kb, and the rmf is (!) 2261 kb.
Summary?
Sizes on brushes dont really matter - face count sets the size.