[Wrong selection size when pasting to another picture]

Hi,

 

I have two pictures and I try to copy a small part of one picture to the other.

 

I did a rectangular selection of 2 cm square of the first picture and copied it but when I pasted it in the other picture the selection was now 4.68 cm square. 

 

I suspect that's because the two pictures have not the same resolution. 

 

How can I keep the size of the selection from two pictures of different resolution?

 

thanks