You would need a converter. You can convert the format (MP4, AVI, MPEG, MOV) to another format. One that may be smaller then what you have. Or change the scale of the video. There are a lot of free ones but the really good one cost a few buck.

There IS an online converter that I used one time to change the format to AVI.

Reduce Video File Size Online, Make Video Smaller (MP4, AVI, MOV, MPEG) | VideoSmaller

You may actually be wanting to compress the file since you're sending by email. But don't compress too much or it turn out awful. Either way you're going to lose some quality.

Try the online converter and see if it does what you want.