The plugin extracts images in this order: the image found in the feed (for the imported article), the post’s og:image meta, images from post content. To solve this issue, please try checking in importing rule’s settings, the ‘Skip Checking Feed Image’ checkbox. It will not use the image found directly in the feed (it think that this is the image that is too small). If this not helps, please try checking also the ‘Skip Checking og:Image Meta’ checkbox.