I'm not 100% clear on your work flow process, but it sounds like you have a continuous video from the game (or even clips from the game), you import them from the camera to your computer, and then within iMovie you're compiling the goal shots. It sounds like this is an on-going project, so you have some compiled shots in an iMovie project, but you haven't exported anything. Is that right?
If that's correct, then the issue is that you're building up a lot of raw footage that's eating away your space. Deleting it and trimming it in iMovie doesn't touch the original files. The problem is that you can't delete those raw files right now, because then iMovie will give you an error, stating that the original files can't be found.
My suggestion would be to adjust your workflow. The imported raw footage can take up a lot of space - import it, export it as an encoded file, and then use that as your new raw footage. If you have the time to do so, removing parts of the footage that you don't need and then exporting will further cut down on the file size. This method will result in some quality loss due to encoding twice (once to encode the raw file, and then again when you've made your compilation); you can prevent some of it by using higher-quality settings with that first encode, but if you set them too high then you might not save much disk space.
Alternately, since you've already isolated scenes that you want to use, you can export those with some high-quality settings and delete the raw footage that they were derived from. Then, when your season is over, bring them all back into iMovie to create the compilation.
The alternative is to buy a large external hard drive. Video work takes up a lot of space.
Hope it helps!
