Render window scaling with two monitors?

I have two monitors - 4K and 1440. My main monitor is the 4K and I use the other one for Aux Viewport. When I render any projects (say at 1920x1080), the image seems to scale up on the 4K monitor's so that it barely fits onscreen (almost like I'm rendering at 4000x3000 or something large). However, if I move the render window to the 1440 monitor while it's rendering, it looks like the correct size. Luckily when I open the final saved image in Photoshop, it displays correctly in both windows.

Now is this a Windows 10 thing? Or a bug? Or is there a feature I'm missing that would fix this?

Thanks!