Images size

I use Summary Card with Large Image, but for some reason, the images are showing up smaller than they should. The original images have 640×300 px, but when published, the Twitter card shows them as 435×204 px.
I’ve tried selecting different sizes on the “Images” tab, and even editing the plugin so my original image size could be an option, but no such luck. They always show up smaller than they should be.
What’s the matter? How can I fix this?
Thanks

https://wordpress.org/plugins/jm-twitter-cards/