It's just an HTML size attribute usage. If you've CSS of any kind altering the width of the input the size attribute will do nothing. Joomlas protostar template sets a width for those inputs. We do not style the login form except for a few specific elements so it blends in better with templates. Specifically the below.
IN: /templates/protostar/css/template.css
Code:
.add-on + #modlgn-username,
.add-on + #modlgn-passwd {
width: 132px;
}
If you've future styling issues be sure to right click and Inspect Element on whatever is styling incorrectly to review its CSS.