Changing the pixel dimensions of an image is called resampling. Resampling can degrade image quality. Downsampling decreases the number of pixels in the image, while upsampling increases the number.

The detail in the original image (left) is crisp, while the detail in the upsampled version (center) is slightly blurred. Much of the detail in the downsampled version (right) is lost.

  Pixel dimensions Size File size
Left image 470 x 1000 2.75" x 4.75" at 240 ppi 1.34MB
Center image 940 x 2000 2.75" x 4.75" at 480 ppi 5.38MB
Right image 157 x 333 2.75" x 4.75" at 80 ppi 153K