They may be similar, but chances are their sonic signals are different. If the wave looks very fat, the file is usually bigger. If there are lots of peaks, but the middle of the wave is skinny or smaller, it will be a smaller file. If you have quieter music, but very high levels, you will have a larger file, so band loudness may not be much of an indicator.
This is not scientific, just based on my own observations after FLACing a lot of files...