Was it whilst exporting that it started to get warm or was it already warming up a good deal when you were just 'playing', composing, editing in iMovie.
It's ok for me when exporting. I can do that at night or on my desk but I'll use it on my lap ( not my chest...) quite a bit when 'playing' in iMovie.
It starts to warm up when you start playing around in iMovie but didn't hit those temps in the 80s until I started exporting.