Why is the "Save for Web" file size way off the actual size?


In Photoshop, the Save for Web dialogue does not give an accurate size for the file. For instance:

The original file size is 2.6MB (checked with Cmd + I on a Mac). When I open it in Photoshop and use File → Save for Web, it tells me that the file size (Original) is 6.73MB. It happens in all my photos.

Why does Cmd + I on a Mac 2.6MB and Photoshop's Save for Web 6.7MB. In a PRACTICAL way, what number should I trust for web design?

It seems that if I upload the file to a server the important number is Cmd + I. Is that right? if so, does the number that gives Photoshop have any PRACTICAL utility?

9/4/2013 8:34:00 AM

Keep in mind that if your operating system supports file and drive compression, then it will show a slightly different file size to you than a software that has loaded that file and is looking at its raw loaded size. The discrepancy is mostly due to how the operating system handles size calculations.

For example, on Windows when you look at the properties of a file, you will see something to the effect of size of the file, and then size on disk. The difference is a number of things, for example cluster size, type of formatting be it NTFS, exFat, FAT, FAT32, so on can lead to difference size measurements for the same file. Hope that helps.

8/29/2013 1:19:00 PM