Hi,
Well, you could add a CSS command to change the width to 100% on the thumbnails.
However, the goal of generating thumbnails is so that your website can provide the image with the exact size so that the download time of the thumbnail for the display of the page as well as the quality of the thumbnail are optimized. The CSS command will only help if it's a "tight fit". But in that case, it might be worth considering optimizing the template or other elements so that there is enough space in the first place. Also, letting the browser adapt the width of the thumbnails dynamically can also cause resizing artefact on the thumbnails, especially if the browser has to upscale the size, so it's not a perfect solution.