Depends on what file format you save in
.psd - non lossy compression - the compression will make you files smaller. If you had many layers the file size coudl actually get bigger.
.jpg - lossy compression - will make you files much smaller
.eps - if you choose no compression, and a preview - file size would be larger than 4.5mb
And also a very small amount of variation based upon your hard disk block size. This difference youi will mostly see on small web graphics.
No one has mentioned the origin of this measurement, so I will...
Once upon a time serious imaging pros saved files in uncompressed 8 bits/channel TIFF format.
It was common for people to speak of the size of an image mentioning its size when saved in that fashion, which is height x width in pixels x 3 bytes per pixel.
Some people still use that jargon today, though the term megapixels has become more common and usable.