"Export > save for web" saves files with higher size than it previews

[RoarkyGer]

I try to save my graphics for the web with lowest possible size but highest quality. I use the "save for web (legacy)" function/save mode. Photoshop shows e.g. 64 kb file size in the "save for web" window, and when I save the file, it has 64 kB, but seconds later it blows up to 74 kB. Why is that? Metadata is set to "without", so what makes the file add 10 kB after saving?

[Stephen Marsh]

Whether Save or Export, file size estimation before saving is just that, an estimate. Compression algorithms make this difficult to precisely predict.

[RoarkyGer]

Hi Stephen, I've been using this method for years since I do a lot of ad banner design. The size shown in "Save for web" was always precise. I noticed the same "error" in the Adobe Media Encoder. It added an extra 15 to 20% to the size of the video file I saved. When being saved you can see the estimated size of the video file in the explorer - it's pretty much correct. Then, 2 sec. later, it seems to "resave" the file and the size becomes bigger.