Pictures Size in the last line of products is different - How can I fix this?

[Maltauschen21]

Hello, 

the pictures of the last line of products is bigger when there are not sufficient products left to fill out the line with 4 products. If there are just two or three, the pictures are much bigger than the others. How can I change it so that the pictures all stay in the same size, no matter how many pictures are in the last line? 

[Maltauschen21]

Here is an example