Well in the post you have the video inside a container with class=rve-embed-container. This is persumably from some plugin (responsive video embed?) you are using. the effcet is to make the video fit in a repsonsive way.
Actaually the theme can also do this for you without tthe plugin, by wrapping it in a conatiner with class=”video-container”.
See http://themeloom.com/2012/10/tip-make-embedded-videos-fluid-in-a-responsive-theme/
So in the forum and the activity stream the video is the fixed width that has been embedded. They need to be wrapped as above in order that they scale with the container they sit inside.
The problem of course is that if a user is embedding the video then you have no control over what they are posting.
So in effect what you need is the plugin you are using fro WordPress posts to be BuddyPress compatible and do the same thing for activity and forums posts. But I don’t think it does.