do i have to use the scale function in photoshop. of can i just change the image size?

i tryed this now on a sphere. i made a simple texture in photoshop. first 2048x2048 then scaled it down to 512x512. and there was a huge difference. then i made the texture from skratch at 512x512 and it was the same.
can you show a render so that i can see the difference ? thanks