without having the file there can be only guessing.

probably higher color depth in that image.

Would a larger ppi do that?

To answer both...all the images are 2048x2048 [Uvah...that's the pixel count] 32bit [moshi....no difference in colour depth].

The 'only' difference is number of layers.....where 30 of them becomes 15meg....and 12 of them become 128meg.

As to what they show...ie painted pixels vs transparency ....essentially the same ratio.

Now....when a layer is 'deleted'  does it still hang around like a bad smell - adding to the size?  [though again, all of them have had layers deleted/added/etc]

i just tried a photo of the same size with 8 bit per channel color depth (=32bit).

12 fully opaque layers = 155 MB

so i still think "real" color depth and transparency are the key. PSD is lossless, but there are some compression algorythms that would make a file smaller if there are areas of the same color. 

I'll strip out all the layers as individual images...then 'combine' them to a new psd....and see what happens.

Well, one way is this: http://www.photoshopessentials.com/photo-editing/photoshop-file-size/ but that refers to Ctrl+J vs. adjustment layers.

I would think that you'd really have to go through all the layers of the files and see if there was needless duplication of image files (actual pixels) instead of an adjustment layer of one sort or another.

The best discussion of it I've seen is here: http://www.photoshopessentials.com/photo-editing/photoshop-file-size/

The essential question is "How good does the image have to be?" and whether beyond a certain point is the law of diminishing returns working. The classic example would be, you working in 32 bit and then outputting to 8 bit.

Also, you can certainly store PSDs you're done in "Save for web" format, if that does not compromise (seriously/perceptibly) the image you're going to use.

Once you delete a layer, it's gone - as far as I understand it.

The whole issue is that these 5 files are the same format...the same origin...the same everything.  There is no visible/physical reason why one would be something like an ORDER OF MAGNITUDE too big.

It's not 'a bit bigger'...it's stupidly bigger.

I didn't click 'compatibility mode' on one and not the other [any diff would be about 10% anyway...not 1000%]  I din't compress one and not the others....I didn't compress any of them.

Simply one MUST be full of some form of crud that it doesn't need.

Here's the thing...grabbed 128meg of layered [12] psd....loaded it into PSP6 [different proggy....just to be sure]....copied out all the layers....all 12....and combined them into a new psd...and saved it.

Size?

14meg.

Round about where I'd assumed it'd be..... BECAUSE it's the same critter as the other 4 images.

It has me stumped....

Could the resolution have been higher for that one file?

Sorry.....dumb question.....already answered.....not quite awake yet.......

As mentioned in #9, I 'rebuilt' the image by copying out each layer as separate images...then recompiled them as a new 12-layer psd.

They have EXACTLY the same layers and even their names and degree of transparency are identical.

The original 12 layer image was  134,077 kb.

The new 12 layer image is 16,037 kb.

I need some sort of 'tool' to find out what the difference of 118,040 kb is due to.

Saving psd files from different programs or different versions of same programs will impact the file size,

in Photoshop for example, it does save a lot of data into the psd, sometimes this data is related to

effects, vectors, masks, etc, but also related to file info RAW / EXIF / IPTC

You can remove the image info (RAW / EXIF / IPTC) by clicking File > File info > Advanced tab

Select the items on the list and click the trash can icon at the top right, click ok and save.

Another tip to make any psd smaller in case you need to send it over to someone, for example,

is just hid all layers and save the psd with the them hidden, this will make the file really smaller.

I hope this tricks help you