It is possible to make it appear smaller.. but not larger. Technically you could create an image for the login screen background with the “user image” as a part of that wallpaper and make the actual user image transparent…
Instead of this image used by default in Windows 11 (and Windows 10), It is the method you need to follow to use the Transparent picture or blank without a picture; You can use a transparent image you like from the internet. I like Rammstein group, so I used their logo. This user picture...