When we’re laying out a design, resizing images is a pretty common practice. However, the more the image is enlarged, the less suitable it is for printing once the resolution drops below 300 ppi.
The file remains the same size in terms of memory because all of the original information captured has not changed, and the total number of pixels remain the same, even though the image dimensions have increased or decreased. In a pixel-based environment, it’s the total number of pixels in an image, together with the number of potential color options available that determines file size—not image dimensions per se.
But there is another approach called resampling, whereby the original information captured is reinterpreted as the file 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 contains fewer pixels than originally captured, and the enlarged image will contain more pixels than what was originally captured. In both cases, though, the pixel size remains the same size.
Resampling decreases the file size as the image is reduced since there are fewer pixels to keep track of, and it increases the file size as it is enlarged since there are more pixels to keep track of than in the original capture.