As shown in the example below, just changing the icon in the same button can result in icons of different visual sizes. In some cases this is a result of the icon shape, but as shown below, in other cases it is just a result of icons using very different margins:
The blue squares shown over the icons are of the same size (21px). The history icon looks bigger than it should in proportion to the label size.
As part of the #wikimediaui_style_guide efforts it would be good for icons to follow a common template.