Photoshop: Very strange file sizes!

Hi!

I've been using Photoshop for some years now. First on the PC, then later on the Mac as I switched six months ago. Currently I'm using CS3.

I upload my photos to some community sites for critics. One of these sites requires the file size to be less than 150kb.

Previously, on the PC, I scaled the picture down (640x480) and adjusted the JPG compression in "save as..." to get below 150kb for the picture. The size of the picture could be previewed in the "save as.." dialogbox, so this was pretty easy.
However, on the mac the file size doesnt match the previewed size. In fact, I'm having alot of trouble getting the files down under 150kb (size still 640x480), even with the JPG quality <5 and the picture looking awful.

I had the same problem the other day, whilst trying to make an avatar for a forum. I found a 200x200 picture, 32kb.
Since the forum demanded the picture being max 70x70 and 20kb, I had to resize it. To I opened it in Photoshop, resized to 70x70 and saved with mid-quality. The picture was now 64kb! How can this be?? Even with JPG quality set at 1, the picture will not get below the 64kb.

This happens with both CS2 and CS3 on the mac. No problems with CS2 on the PC.

Doesn't "save for web" delete the EXIF data?

I need this data to document the shutter time, aperture etc

Just saved for web, and the EXIF in the new picture weren't present, but the file size was correct.

Is there any way to keep the EXIF data?

Yes, it's on the original. I do however need the data on the copy.

Looks like I'll have to edit in Photoshop and export using Lightroom. This seems to work.