The .tx format allows compression of the .tga files. By default the TX Converter generates uncompressed files which can be pretty darn big as you've discovered, but it also allows you to generate compressed files. That's worth experimenting with.
I wonder... now that we've got the source code, could we make the game read .tga files instead of .tga.tx? If we could, would we want to? Or would we lose the ability to compress the files? Would .tga files load faster or less fast in the memory than .tga.tx file? I really don't know...
Because the .tx is simply a container file for the .tga, the tga would probably load faster as all the .tx on the end does is add a step to the loading process. Additionally, if the .tx files are compressed, the engine most likely has to decompress them in the memory before displaying them, which is yet more load time. I doubt the change would make a significant difference either way, but it would be more convenient for modders, as we could just save out projects as .tga with gimp or photoshop, rather than having to use a poorly constructed conversion tool