imovie export 10x bigger than source file???

If you export at Best Quality (pro res) you will get a .mov file at least 4x bigger than if you export at any other setting with comparable compression rates. That's because of the much higher bitrate and less compression of that format.

If you import a highly compressed video into iMovie, iMovie needs to unpack it for editing, and when it exports it out it may not be able to compress it as much as when it went in. Thus larger file size.

It is not uncommon to see larger file sizes when exporting from iMovie. This may or may not be relevant unless you have severe space limitations (in which case an external storage drive might be appropriate) or you want to send it in an email or post it on a website.

-- Rich

One more thing.

If you don't want the default Mbps (megabits per second) that the other share out options provide, you can do a Custom quality setting in the share out dialogue box. Click on Custom and you will see a slider bar to set the Mbps. If you slide it back and forth you can see the affect on the file size by noting the file size indicator beneath the screen in the share out box. Keep the setting around 8 Mbps and there should be little noticeable difference in quality, but you can go lower if you want a smaller file size.

-- Rich

You are welcome. Have fun.

-- Rich

iMovie will export the movie at the frame rate that you set for the project.

You set the frame rate by placing a clip with a known frame rate as the first

clip put into the timeline of a newly created project. So if you put in a 30

fps clip as the first clip placed into the project, your project will be set at

30fps. (You can later delete that clip if you want, without affecting the frame

rate setting of the project.) The project will export at 30fps, not 60fps or

any other arbitrary frame rate. The project also will be set at the same

resolution as the first clip, although you can change the resolution at the

time of export.

The biggest determinant of file size and quality is the bitrate of the codec

used to create the video.  The higher the

bitrate the better.    Before importing into iMovie try opening your video clip

in the free download, MPEG Streamclip. Check the bit rate by clicking on

File/Show Stream Info in the Streamclip menu. Look in the Video Track heading

and note the bit rate, frame rate, and file size of the video clip before

exporting from iMovie. Then export the clip from iMovie without dumbing it down

by lowering the resolution or increasing the compression. After exporting, open

the exported clip in Streamclip and compare the bit rate, frame rate, and file

size with the pre-exported clip.   I am guessing that the one that you exported out

of iMovie has the higher bit rate, higher file size, but the identical frame rate versus the

original clip.  The higher bitrate explains the larger file size of the export. Sometimes

dumbing it down doesn't entirely override the affect of the higher bitrate.

In summary, you get a better clip coming out of iMovie than you got going in.  In my

view that is a good thing.   With the cheap price of storage these days file size is fairly irrelevant. 

Quality is most important.  The frame rate will be the one that you set for the project.

-- Rich