OP, trying to email video is generally a challenging prospect. Better to post the video to youtube, vimeo, dropbox, etc and email a link. Then, you don't have to overly worry about whether a particular email client can actually receive the video.
BUT, if the goal is file size, export it at high resolution out of iMovie, then run that through Handbrake(HB). To cut file sizes down, you can do various things in HB. For example, moving the "constant quality" slider up (so the quality number gets bigger) will cut the file size of the final render (by sacrificing some detail quality). Try 21, then 22, maybe higher, and see if the quality is "good enough" (in exchange for yielding shrinking file sizes).
Another option is to reduce resolution. If it's 1080p, do you need it to continue to be 1080p? Consider 720p or even SD resolutions. It sounds like you've shot it at 720p, so consider a switch to SD resolutions. As you cut resolution, you'll cut file sizes.
Both?
The game you're playing is tossing out detail to yield a smaller file size. How much of a loss of detail you can tolerate will directly relate to the resulting file size. Reducing resolution and increasing the constant quality slider will yield a smaller file. Reducing resolution more and further increasing the constant quality number will yield an even smaller file. Experiment until you find the "good enough" (quality) tradeoff.
Lastly, it sounds like the video has already been shot. If there is a lot of camera movement, all that background detail (moving) yields a lot of data in the file. For example, if it's all shot with someone holding a camera instead of using a tripod or stand of some sort, even smallish movements that basically has everything the camera "sees" moving is detail to store in the video file. If you have this problem and a re-shoot is possible, use a tripod or similar to stabilize the camera. If the background is more stable, the file size will be smaller.