How can I resize images of grid view to fit the entire frame

[costispaps]

Hi

I am trying to figure out how I can increase the size of the product image in grid view. The second image that appears when hover is the right size. It is the first image I am struggling with. I have attached images to show the difference in sizes. The site is at https://www.arodou.gr. It runs Presta version 1.6.1.7 and it has a custom theme although I think the product grid image size code is the same with the bootstrap theme.

 

Thank you