Actually its not the detail in the texture that affects tga size, its the BPP, image type and image size. Make sure they are all 32 bpp and make all black and white textures greyscale format. You could also lower the colors of indexed textures.
TPL files are a certain equation smaller than tga files, so a completely black tga would be the same as an indepth face drawing.
Ah. Thanks for the info (Edits post)