A bit more on the same lines:
My guess is you have been measuring in real world units, inches or millimetres. So the strap is say 20 mm @ 300 pixels-per-inch (ppi) and the buckle is 20 mm at 150 ppi
Make sure the size in pixels matches. The ppi does not matter too much, except for quality purposes. Drop the buckle into the strap image and providing the pixel size match, the image keeps the ppi of the strap.
My guess is you have been measuring in real world units, inches or millimetres. So the strap is say 20 mm @ 300 pixels-per-inch (ppi) and the buckle is 20 mm at 150 ppi
Make sure the size in pixels matches. The ppi does not matter too much, except for quality purposes. Drop the buckle into the strap image and providing the pixel size match, the image keeps the ppi of the strap.