If you did it properly, then the image in your drawables-mdpi should be 1/2 the size of the image in xhdpi.
What you're seeing on the screen is that a typical mdpi device is 320 px while a typical xhdpi is 720 px. So if you made two buttons, each 1/2 the screen size, then the xhdpi buttons should be 360px. Following "procedure" means half sized 180px buttons in mdpi. But on the mdpi screen 2 buttons would be 360px, and not fit. To get them to fit you would need to reduce them 30 px, or about 13.33%
Sounds like the screens you are looking at may not be typical screens, but the root of the problem is that you are adjusting the image size in both the drawables and in the values folders. You need to redesign your approach to handling different screen densities - pick one or the other, but probably not use both unless you have a very unusual use case.