BleepingComputer.com: Problems with Preview

I'm cropping images in Preview. They are roughly 900kb each to start, .JPG format, and I'm removing about 30% to 50% of the image area. Then I go to save the edited file (still as a .JPG), and suddenly the file size jumps to 1.8MB or more as displayed in Finder. What is happening? How can I set Preview to NOT do this?

Okay, not to bump the topic, but I've got some test data to lay out. Someone suggested just using "Save As" and changing the file type to a PNG. Long story short, it didn't work.

The images I'm working with are of general two types: full color images (photographs the size of a sheet of letter paper, scanned in) and grey scale graphic with text (diagrams, also the size of a letter sheet of paper, scanned in).

the full color photograph will hereafter be called Image1
AND
the grey scale graphic will be called Image2

Image1 begins as the unaltered scan, format JPG, file size = 1.0MB.
Image2 is also an unaltered scan, format JPG, file size = 596kb.

Image1 saved as a PNG becomes... 11.6MB.
Image2 saved as a PNG turns into... 4.7MB.

So... uh, that's not working.

Just for the hell of it, let's try from the intermediate step of the cropped JPG with mysteriously inflated file size:

Image1 with some of the border cropped in Preview was saved as a JPG and became 1.9MB. Saving this file again as a PNG makes it 10.8MB.
Image2 with even more border cropped off was saved as a JPG and became 868kb. Saving this file again as a PNG makes it 3.4MB.

What the hell, Preview? There's got to be some crazy metadata getting thrown in there, or a bunch of profiles or something... I'm still lost.

Have you tried Tools menu > Adjust Size instead of Crop? For some reason the Crop option in my Macs Preview is greyed out when I open jpg files.