Digital Light & Color

I never thought much about this until I noticed a question in a different forum about whether one could save a file to a specific megabyte (MB) size. (Seemed to be a strange thing to want to do.) But I recalled that one of the data points in PW's Resize transformation was File Size, recorded in MB. So I presumed that through trial-and-error percentage resizing one could observe the New, or resulting, file size.

I loaded one of my images that Windows Explore said was 3.23 MB into PW. When I called up the Resize transformation on this image, it said the File Size of the image (before any resizing is applied) is 17.2 MB. That sent me down a totally new path.

What do these two wildly different file size measurements represent?

If the file is a JPEG or some other lossy format, it gets compressed so the file size on disk is usually less than the file size in memory. Regardless of the format, there cab also be some overhead for headers, metadata such as EXIF, and color profile.