The file size of 48MB they refer to is the size that is circled in my example. If you are in doubt if that is the correct file size, click on the little arrow to the right of these numbers... you'll see a drop down menu where you can choose "Show..." and then you'll see, among others, "Document Sizes". That is the correct uncompressed file size they refer to. Beware, if you are using multiple layers in your image, the second number shows the multiple layered file size.

We do not see things as they are. ǝɹɐ ǝʍ sɐ sƃuıɥʇ ǝǝs ǝʍ