Hi Nadia,
Yes, that file size is indeed too big -- may I ask which audio file format you have used for your Little Math sound set?
If you're using the wave format (*wav), a possible solution for you is to convert these to *.mp3 format as this can greatly reduce the file size of each audio file. The file sizes of each file attached to your LM sound set (or even for icon sets and LR files) greatly influence the resulting file size of your LM (and LR) file, so converting your audio files to a more compact format can definitely help.
** Little Reader and Little Math can support .mp3, .wav, and .wma audio file types
Also, the LR and LM file formats (*.cat, *lmtm, etc.) already work like a file archive, so when you use LR or LM's Export function, the file size of your LR or LM file will be slightly compressed.