iMovie is a video edit app, meant to work with miniDV => dv codec => 13GB/h...
(720x480pixel/8bit color info/30 frames per second + audio ... do the ridiculous math...

)
you could compress with other codecs => no work with iM ... that simple, no workaround, dv is internal file format of iM (or AIC, which has 4x the size.. for HiDef)
after editing, you can choose in iM's expert settings any other QT codec.. but: playback codecs (as h264) are for ... ehm, playback only.. they are lossy, means: re-converting such coded files drops pic quality immediately...