Main Image thumbnail is displayed in low resolution

The main image of my listing has a resolution of 2000×2000 and looks very clear and crispy on the product page itself. What I noticed though, is that the thumbnail for example in the search results, is rather blurry looking compared to products of my competitors. I double checked and downloaded the thumbnail picture of my product and competitors, and indeed, the resolution is only 300×200 pixels on mine, but 600×400 on the competitors.

I also tried bigger resolution (about 4000×4000), but that also did not help.

Has anybody faced similar issues? Is there a specific resolution the main image has to have for it to display high reolution thumbnails?

Thank you in advance!