AMP pages that have structured data need at least 1200 pixels wide marked-up image. But the NON-AMP version only need 696 pixels wide image.
Any reason why the mobile search result needs such a large image?
Doc: Link to AMP DOC
If I were to make a guess I would say it is to ensure the images are crisp when viewed on a high DPI screen. 696px might look fine on something that has a fairly low DPI, but on a higher quality screen with a high DPI 696px may be fuzzy and probably show some JPG artifacts. Where as a 1200px image scaled down will remain crisp on all displays.