Photoshop: Problems with copying & pasting the image

Photoshop: Problems with copying & pasting the image like this:
1) I open the image with certain sizes
2) I make a new image (document) a little bit bigger
3) I copy the first image (1)
4) I paste it to this new image (2) >
It does not fit on it: It is not any more of that original size but seems to be much larger - Why?