The reason is that cropping, changing the canvase size or using trim will get rid of the pixels outside the new canvas size. What you need to do is:
METHOD A
1) create a new document to the final size you need.
2) Link all the layers together in the dopucmetn with the artwork
3) hold down shift and drag all these layers to your new document
METHOD B
An alternative method you would put a vector layer mask on a set containing your artwork, but this method would leave a larger image with tranparent background when you place in lets say InDesign or AE. You would also have to turn on the blending option for Vector mask hides effcts
I think it is great that Phothsop crops and gets rid of extra pixels, but would be nice to have an option to retain the pixels in a floating layer, so that effect such as Bevel Emboss don’t look differetn after cropping.