Hi,
it sounds like you are taking a jp2, jpg, ECW or MrSid file and moasicing them to a new image format but not specifying the Compression value in the environment varaiables. Before you output the raster select the Environments button, then select Raster Storage. Finally the Compression setting. Here I would suggest you pick JPEG2000 and set the compression at 75%. This will proabably give you a mosaic of comparable size to your original images combined.
If you are only going to use it in ArcMAP then you will get even more size reduction if you use the new Geodatabase format Mosaic Dataset. This leaves the images in their native format but builds pyramids (overviews) over the top of all 5 of you images. You could call it a pseudo mosaic as no new image is created but they are all displayed together as if they were mosaiced.
HTH
Gordon