So after a lot of fiddling around with the parameters I eked out the best possible quality I could whilst still being under 10mb for a movie file I was exporting. I got it down to 8.8, it was on a USB stick and given to the person who wanted to attach it.

After attaching it to a new message it jumped to 11.1 megabytes. I thought it might be a difference between the filesystem on the Macbook and the fat32 system on usb but I tried copying it to the desktop and the size remained the same. Why does it do this? When I use Mac os mail to send attachments, they remain the same size as they were before I clicked attach. What is outlook doing?