This css in https://github.com/discourse/discourse/blob/master/plugins/lazy-yt/assets/stylesheets/lazyYT.css appears to be the culprit.
.ytp-thumbnail-image {
position: absolute;
width: 100%;
height: 100%;
object-fit: cover;
}
The onebox div that would restrict the dimensions is missing the lazyYT-container class on the review queue and edit history screens changing the meaning of 100% for those instances.