Different thumbnail sizes on two pages with the WordPress gallery

My theme let’s me have one page with a side bar and one without. The perfectly square thumbnails are the right size on the page with the side bar (https://a4jp.com/exhibitions-2011-tsunageyou) but are a little bit small on the page without the side bar (https://a4jp.com/inspiration-gallery-0001). How do I get the thumbnails bigger on the page without the side bar in the gallery?

Glen