I saw this last weekend on a friend's PC running Windows. I clicked the maximise icon in the lower right corner of the video, the video appeared to change resolution but the space it occupied did not change, in other words it was more like a zoom function. It was not constant either, in that it happened sometimes with some browser tabs whilst other youtube videos in other browser tabs maximised normally. Internet Explorer was being used, by the way. So, my opinion is that it is an anomaly in Youtube's use of HTML5 code, now that they are moving away from Flash and as noted above, it has been seen on a m/c w/o Flash installed.