Just so you know.. I've been a pro editor for 10+ years.. I deal with files, compressions, trancodes, etc daily.. I'm here to help out as best as I can!
back to your questions:
It has to do with the codecs that are used to compress the files..
Quicktimes, .mp4s, .mpeg, wmvs files can be created dozens of ways.. some have huge data rates, others are small.. its based on usage..
example.. Broadcast files have data rates of 220 mbps and up, with 170 mbps being the low end..
compare that to a movie download.. maybe 6mbps.. trust me.. a 1080p movie that is 10 mb per minute (file size) is garbage.. the 1080p I download are usually around 4gbs+, that is 33 mb per minute.. they look decent.. the better ones I have are more like 15gbs per movie, which is 125 mbs per minute.
Speaking of.. movie rips are usually compressed more so people can download them quickly..
Now, what is the exact format of these movies? You said HD cam, but are they AVCHD? Mpeg or quicktime files? good info to know to help you.
Heres a test.. do you have mpeg streamclip?
http://www.squared5.com/
Its faster then FCP..
throw a file in at and choose H264 as the codec.. but dont constrain the bit rate.. leave it on auto
let it rip.. compare that file to the original media.. should look pretty darn good.. But if the file size is too large.. then throttle the data rate down.
You can also export right out of FCP this way, just dont set a data rate.
This is to give you a bench mark as to how they look, and how large the files are.
And honestly, storage is cheap.. a little external 1tb is $100.. these are your home movies right? Then you should store them at good quality, and always hold on to the original tapes if you are shooting on tape... or as I do, keep the original footage on a drive.