copying layers from one canvas to another

It all about the numbers of Pixels layer have.  Layers can have any numbers of pixels they can have the same amount of pixels a document canvas has or have more or less pixels.  A layer bounds can have any  aspect ratio and be any shape.  Resolution has nothing to do with a to do with the number of pixels a layer has.  Resolution is a printer setting that set the size Pixels will be printed.  Resolution is also used to calculate the numbers of pixel to deals with when you use other ruler units with Photoshop commands like inches, Centimetres etc rather than Pixels.    Photoshop is a Pixels editor.     Image details are stored by pixel data.  You can get a images taken with different camera a 8MP camera. 16MP camera a 24MP camera the all can take a picture of the same subject at the same distance withe the same focal length lens  The images will all have the sane subject but the images  will have a different number or Pixels.  Some Images have more details than others.  If  you want to user the image from different cameras in a composite you need to scale the images to have the same amount of pixels. So all images will have the same scale. 

Hi, on the preferences general double check you dont have adjust size during pasting selected

Are the documents the same resolution?