Featured image on mobile is shown as mini thumbnail

I recently got a problem with my website on mobile phones. The featured image for all of my posts are shown as thumbnail sized pictures, but the correct size is shown on laptops. I am not sure how to fix this problem. My theme soledad doesn’t have an own mobile photo size option, or at least I cannot find it. Please help me to solve this problem.

www.heartofsaffron.com

Kind Regards
Iselin