Generally the categories thumbnails size can be set in Ecwid control panel ->System Settings → Design → Thumbnail Sizes page.
I have checked your store and, as far as I can see, you have Category Thumbnail Size set to 223 px, while images on [url]asos.com[url] are 230px x 180px.
So Ecwid takes your category images and fits them into a square of 223px x 223px, changing their proportions accordingly. If you have a narrow image (like https://dpbfm6h358sh7.cloudfront.net...
, for example), its height will be 223px, while the width will be still little, because Ecwid will create a thumbnail with the same aspect ratio as the original image, fitting it in a square of 223 px size.
To have your categories images of the same size like at [url]asos.com[url] you need to change image proportions with some graphic editor (the easiest way will be to make them all 230px x 180px) and then set Category Thumbnail Size to 230px in Ecwid control panel.
Please note that you will need to re-upload the edited categories thumbnails to your store.