Aside from adding custom CSS to change margin and/or padding, the quickest way to increase product images sizes is to reduce the number of products per row via the bundle's Design editor options. This will give each product more space - allowing them to increase in size to fill the extra room.

Select your bundle, click on the Launch design editor button (top right of your bundle).

Pick you template, and look for "Maximum products per row" under "Product Options". Find a configuration that works for you.