The mrs file isnt 18mb because of not caulked faces.
Its because of the Textures.
If bigger then 512x512 you mostly will have realy big problems with post developing.
(Adding External Models, use alphas)
Resize em to 512x512 if your textures are bigger than 512x512.
Also, be sure to not use bmps.
Bmp is the worst Image Type ever, has alot of info that srsly no one cares about or needs.
Try to use PNG or JPG, i only recommend JPG with a good quality ratio.
(Wich Paint doesnt has, so dont try to resave the textures with paint to jpg...)
