Okehe ...
I saved a test tga as 24 bit without RLE with Photoshop CS (8.0.1) and Paint .NET. File size is exactly the same.
Of course ... it could be possible that Paint .NET adds informations too. But my believe is that the file size will be the same with any other application.
I'm not sure if wow accepts 16 bit tga files. Maybe the original file is saved as 16 bit?
[edit] 68 kb and 100 kb really looks like 16 bit/24 bit
(100/68)*16 = 23,53