why does iPhoto only import digital photo files at one-half or less of the original jpg file size?

My 24" 2008 iMac, running 10.5.8 and iPhoto '08 (300 GB harddrive, 67 GB avail, 2.8 GHz Intel Core 2 Duo, 2 GB 800 MH sdram), only imports my digital photo images directly from the camera at about 1/2 or less of the original file size. I thought it might be my 12.2 mgp. Samsung camera, until I just a 14 mpg. Lumix, and its doing the same thing. A 5 mg. images imports at around 2.4 mgs... a 14 mg image imports under 6... Its always done this, but until I bought the new camera, I assumed it was my peripheral issue.