Hey Greg
What video player are you using?
By downloading .mp4 and .webm I invoke the browser's inbuilt video player, and by default I can switch to full screen. Alternatively you can access the browser video controls with javascript and do the same. Support for legacy browsers needs to be built in, which may not support full screen.
ps - someone more easyXDM aware than me may be able to point you to a better solution. Have you for example looked at re-writing the attributes via the intermediate.html page?