Image on external monitor zooms unexpectedly

Mac Book Pro 15 16g memory Late 2014

When on either non Apple external monitors the opened window zooms to a size that is to large to see the entire window My larger monitor is a high resolution (ASUS)i monitor connected with a Call Digit Thunderbolt adapter. The other monitor is a Deell concted directly to the Mac Book with a Apple Thunderbolt adapter. Do you have any Ideas on how I can correct my problen. Very grateful!

Marc