You can try to activate any of the default themes (twenty fifteen, twenty fourteen…) to see if this is indeed a theme issue.
You mention that your plugin is set to show sizes of 125px × 125px but I’m not seeing any size being set by the plugin at all (by examining your page). It simply displays the original image whatever size that may be. There’s really nothing on the theme’s css that could be messing with your ads sizes. You may have overlooked something on your plugins setting.