Copy the HTML code from the original thread to the new one. That's why there is a difference here. You can see the HTML code when you click on the little <> at the right end of the tool bar above.
The html code in your original post is a correct "iframe" call:
<p><iframe width="560" height="315" allowfullscreen="allowfullscreen" frameborder="0" src="https://ogrforum.com/video-embed/t/0/c/153603996457099950/p/153603996457262803"></iframe></p>
Note the width is 560 pixels and the height is 315 pixels.
Here is the html code from your copied thread:
<p><video width="300" height="150" controls="controls"><source src="https://ogrforum.com/fileSendAction/fcType/0/fcOid/153603996457099950/filePointer/153603996457262803/fodoid/153603996457287575/stream/true/trim.6160BCEE-A1A5-43EB-8EF6-03FF0DF822E1.MOV" /></video></p>
Note that it is not an "iframe" call, it's just a "video" call, which is not the best way to do this. Also note that the width was only 300 pixels and the height was only 150 pixels. I changed them to the 560/315 sizes and it enlarged the video thumbnail OK. The video plays OK here now.
However, you really should use the "iframe" code here.
If you prefer not to get into working with the html code, then I would suggest you re-upload the original video instead of copying it.