Hi,
Well, there are several ways:
- you could reduce the width of the thumbnails under the "products options" tab of your menu item (or module) via the Joomla menu manager (or module manager)
- you could reduce the margin configured at the same place
- you could reduce the number of columns at the same place
- you could have a combination of these so that the space available for each image corresponds to the size you configured for the thumbnail itself.
- you could add custom CSS to force the image to fit in the space avalaible.
What to do will depend on what you prefer to do.