Hello 
If I am correct than on your two shared screenshots in OP. The first one is shows a YouTube video shared with embedding code (I suspect it because the video author avatar is appears). The second one is shared with link.
On the first one the embedding code contains such parameters like size etc… I think only the second (shared link) video goes through the Discourse magic process to make it lazy loading and set the size etc…
On my forum it was pretty common to share embedding code instead of link. I had to warn users to share only the link as we have lots of topics with music, movie etc… and a tons of shared YouTube videos. If they shared it with embedding code the whole topic will slow down because when you open it it will load the videos.