Okay, I think I figured out what's causing it. These same images are already in a slideshow in the "Phone" version of my site. So that's the size they are scaling up to. If I copy the image to a new location and add it into the slideshow, it recognizes it as a new asset and scales it properly.
Now I'm wondering if there's a way to allow them to behave independently, or if I'll just have to create a separate folder of images for each site layout...