Hi Michael,
Thanks for reaching out.
Do you mean the size should be exactly the same? Because I compared and the aspect ratio is the same, and the image is exactly the same with just different sizes because of the container, and it responds.
Please add this CSS to your Theme Options > CSS change its size
@media ( min-width: 980px ) {
.blog .x-main .hentry>.entry-featured, .search .x-main .hentry>.entry-featured, .archive .x-main .hentry>.entry-featured {
width: 34%;
}
.blog .x-main .hentry.has-post-thumbnail>.entry-wrap, .search .x-main .hentry.has-post-thumbnail>.entry-wrap, .archive .x-main .hentry.has-post-thumbnail>.entry-wrap {
width: 66%;
}
}
The 34% is for the image while 66% is for the content. If you increase a value then make sure to decrease the other one. The total should be 100%, example, 40% and 60%.
With responsive layout, the image size is controlled by its layout dimension. The only way to change it is by changing the container’s size.
And with the existing custom CSS in your site, then I think you’ll able to enhance and customize the given CSS further 
Thanks!