HDMI Monitor on Mac OS vs Windows 8.1

I have a mac pro (Late 2013) and I have three monitors hooked up to it. They are an Apple Thunderbolt monitor, and LG HDMI monitor, and a Samsung DVI monitor. I run the latest Mac OS (Yosemite 10.10.3 as of this writing), and I also run Windows 8.1 on Bootcamp. This setup works great on both operating systems, but I have noticed that on Mac OS, the HDMI monitor puts the image on the full extents of the display, while under Windows, the same monitor scales the image down so that there is a 1" black border around the screen.

Does anyone know why it does this, and what can be done to make the windows side use the entire HDMI screen?

Many thanks,

Matthew