In the standard theme the first and third columns in the "Allow role assignments", "Allow role overrides", and "Allow role switches" tabs under "Define Roles" are smaller than the other checkboxes in some browsers.

Styling behaviour seems to be browser and OS dependent as described here:

http://www.456bereastreet.com/lab/form_controls/checkboxes/

so it doesn't happen in all browsers, but on my Mac (Snow Leopard) with latest Chrome the boxes are quite a bit smaller.

The rogue style is in theme/standard/style/admin.css:

#page-admin-index .explanation,

.path-admin-roles .cell.c1,

.path-admin-roles .cell.c3 {font-size: 0.7em;}

Looks like the .path-admin-roles class is being used to style something else but isn't specific enough so it's being applied elsewhere.