I am using YakYak 1.5.9.

When someone sends me a video, YakYak displays the thumbnail in the chat and when clicking on it, it only opens a higher resolution version of this thumbnail (https://lh3.googleusercontent.com/[some long ID]).
When using the official Hangouts web client, clicking the thumbnail opens the video in the browser instead (https://r5---sn-4g5edne7.googlevideo.com/videoplayback?[some GET parameters]).