AVCHD footage is approximately 2 Mb per second which is pretty good compression, especially for HD content. Unfortunately, you can't edit raw AVCHD .mts files on a Mac, even though iMovie 08 and Final Cut Express 4 both claim to "natively" support AVCHD. What they actually support is transcoding of AVCHD footage from the camera to Apple Intermediate Codec (AIC) format on import. AIC files are optimized for realtime editing and they take up about 8 Mb per second* of footage, which uses up disk space pretty quickly.

* These numbers are just based on my personal observations, and compression ratios vary due to the variable encoding rates, but they are in the ballpark for my Sony HDR-CX7 camcorder capturing AVCHD footage at 960 x 540 resolution through iMovie 08.