Splitting a long clip in iMovie into smaller and separate .mov or .jpeg files

I have ten hours of 16 mm home movies that my grandfather took in the 1930's and 1940's. I had them digitized but the resultant files are quite big--they range from 4 to 18 Gigabites. I've tried to use the split clip (Command B) to separate them into different files but I think what happens is that the original file just has blank spaces where I've edited it and nothing really has been deleted from the file. That is, each file takes up the same amount of space, regardless of how I trim it down.

Does anyone have any advice? I've watched everything I could on Youtube about file management and even paid for some training videos, but nothing seems to make the files themselves smaller on my computer. Most of the on-line training videos assume that you are making videos of many different smaller computer files (clips), instead of working with one large file that you want to split into smaller "clips".