There is a setting under Windows 8.1 to re-size icons and generic Windows things, even browsers are affected.
The charms menu on the side (mouse in top right or bottom right corner) -> Settings (Gear icon) -> Change PC Settings (at the bottom) -> Display
Or
Control Panel -> All Control Panel Items -> Display
"Change the size of all items" set this back to 100%, if you use more then one monitor you can choose which to do this to or run them all at the native resolution.
Microsoft's attempt to be user friendly on 1920x1080 screens that are 10-15 inches in size.