Breadlife theme is legacy not responsive theme. But it's div based not with table layout, but without bootstrap it will be not displayed correctly on mobile devices.
Gallery plugin is new one (for version 2) and so gallery template is bootstrap based template.
With legacy theme there is special back-compatibility css loaded, so bootstrap grid classes are in action (col-md-3) and images are displayed correctly with themes too.
If you need to change image size, you need to create change gallery template (in theme folder) and change classes responsible for image width: span3 col-xs-6 col-md-3
To make this theme responsive, you need to add bootstrap support there (but it could break other things) or at least some responsive grid. By updating theme according new standards (theme.xml is available) that backcompability css will not be loaded.