What is resampling?
Resampling is when the original information captured is reinterpreted as the file and is enlarged and reduced. Here, the resolution is maintained at the required amount, say 300 ppi, but as the image is enlarged or reduced, the original captured information gets reinterpreted (resampled) by the computer. An attempt is made, usually a pretty good one, to maintain the way the image looks, but the colors in the pixels will all have changed, since the smaller image will contain fewer pixels than were originally captured, and the enlarged image will contain more pixels than what were originally captured. In both cases, though, the pixel size will remain the same size.
Resampling decreases the file size as the image is reduced since there will be fewer pixels to keep track of, and it increases the file size as it is enlarged since there will be more pixels to keep track of than in the original capture.