For the search results try to install stylish. Create a new style using this:
ytd-video-renderer [use-prominent-thumbs] ytd-thumbnail.ytd-video-renderer {
margin-right: 16px;
max-width: 36px;
min-width: 220px;
-ms-flex: 1 1 1e-9px;
-webkit-flex: 1;
flex: 1;
-webkit-flex-basis: 1e-9px;
flex-basis: 1e-9px;
Just copy and paste. It is not perfect but it is better than these giant thumbs.