Ubercart uses the "Image Styles" (Previously known as ImageCache) to control the display of product images. When you first install Ubercart, we creates a number of "presets" for how the images will display in a fresh install of Ubercart. Which define how product images are displayed in various situations. However, these image styles can be customized very easily.
You can modify Ubercart's image presets, or create your own! Just go to the Image Styles settings page (yoursite.com/admin/config/image-styles) and you will find the list of all the presets along with details of how each preset displays images.
The preset that controls the image display on the product page is called "uc_product"; the "product" preset defaults to scaling the images to fit in a 250 x 250 pixel box, while preserving the original image's aspect ratio.
Override the "product" preset and modify the settings to fit your needs. Be sure to submit your changes by pushing the "Update style" button. It's also recommended to clear cache (yoursite.com/config/development/performance) to clear out any images that were built using the previous values.
Note that by modifying the other presets, you can also control how images appear in product teasers, on the cart page, in the product list page, etc.