Invalid thumbnail

When the “Replace Youtube videos by thumbnails” function is enabled, thumbnails are replaced. However, the sketches are too small. For example, for a page https://ldesign.studi (the most recent block at the bottom) the thumbnail for the video looks like https://i.ytimg.com/vi/w5uVCsDXUw8/hqdefault.jpg . Why is the sketch so small?

The page I need help with: [log in to see the link]