are you using windows 10?
your display scaling or something like that is up, most likely happens on windows 10 laptopsi don't remember how to change it, but yours is probably at like 110% or 125% or something when it should be 100%
most likely this, this might helphttp://www.pcworld.com/article/2953978/displays/use-windows-10s-individual-display-scaling-to-perfect-your-multi-monitor-setup.html