PS won't resave file, says too big, but I didn't change the size.

[photoboydc]

Here's something odd...I just opened a file and changed the physical dimensions (smaller) without resampling. I changed nothing else. I tried to save that one change back to the original PSD file I got (after many minutes) the dreaded prompt saying it was too big to save. But it wasn't too big to save when I saved it originally, obviously! The file was created/saved in Creative Suite 5; the resaving attempt just now was in Creative Cloud. Should that make a difference?

So I resaved it as a TIFF. I have just checked the sizes of the original PSD and the new TIFF. Pixel dimensions are identical, as they should be. No layers added, bit depth not changed, etc. But the original PSD shows up in the thumbnail summary as 2.06 GB. The TIFF is 3.37 GB. Same file. What gives? And can anyone explain why PS, after all this time, is unable to know at the beginning of the save that the file will be too big? How can it not know the complete file size?

[JJMack]

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[photoboydc]

Let me add a few possibly pertinent details...I am on a 2015 27" iMac, tons of open memory, upgraded to latest Sierra. The file being saved (or not) was on an external drive with 2TB of free space, connected via Thunderbolt. I can't think of any system constraints that would result in this Adobe weirdness.

[Test Screen Name · Accepted solution]

The largest PSD you can save is 2 GB. Since apps don't always agree on what a gigabyte is, that's 2-ish gigabytes. The size of the file will depend on some compression, which isn't known in advance. And on metadata, which will increase... your file size is obviously borderline. Or Photoshop has done something to store lots of extra info.

Happily, Photoshop offers the PSB (B=big) format for larger files, so once your files get big, use this instead.

[photoboydc]

Thanks. There is compression in PSD's? Didn't know that. I also find the 50+% increase in file size in this instance going from PSD to (uncompressed) TIFF to be remarkable.

[Test Screen Name]

I believe there is some mild lossless compression. Maybe just runlength compression (which would compress exact repeated bytes, especially good for masks).