Screenshot file much larger dimensionally than original
Screenshots taken on a Retina display will be twice the size as screenshots taken on a non-Retina display.

Most software such as Preview will compensate for this, however I'm unfamiliar with Paint X and doubt it is correctly showing the image rather than the image being 'wrong'. Try using software optimised for a Retina display. 

If you want take screenshots for non-Retina displays, you can use RetinaCapture. Note that these screenshots at 100% on Retina displays in software optimised for Retina will be smaller than real size.