What?
I don't use iMovie, but I have edited 75GB of SD video in FCP and Avid Media Composer on my iBook without hiccups, except when I wanted to apply effects or such things.
Last year I edited 250GB of SD video on my 2009 MBP.
At work we have a quad-core 2008 Mac Pro with 4GB RAM handling four TB of SD and HD video without hiccups. The years before that, we used a quad-core 2006 Mac Pro with 4GB RAM to handle 10 TB of data per project.
How do you come to such a ridiculous conclusion?
PS: Compressed footage is harder on the CPU during the editing process, thus MacBooks (for example) are not so good at handling MPEG-4 footage from an AVCHD camera. Thus iMovie normally transcodes the video during import to a .mov file using Apple Intermediate Codec (AIC) as a proper editing codec, which is less taxing.
The 75GB of .mov files are most likely using AIC as codec, or maybe just the DV codec, as VHS material is lower than SD quality (320 x 240 to be exact, SD is at least 720 x 576 in PAL). 12GB of DV (or AIC) encoded can store one hour of video, thus 75GB are good six hours of video.
75GB of highly compressed video would be 75 hours of video, or even more.