The size of the wave file is larger than that of the mp3 file, why not use text-to-speech freeware panopreter to batch convert text files into mp3 files?
These WAV files were only "temporary" files on my PC, Camtasia of course converted them into MP3 when encoding the final video.
DSpeech also supports MP3, but since I have enough free disk space, I prefered the uncompressed files as input in Camtasia.