A problem saving a large image.

Yeah, that's what I figured.
Its too bad really, but it wouldn't be the first (or last) time a superior technology fails to gain popularity.

I can't belive how easy it was. Changing the value (slider) even from 12 to 11 when saving changed the image size from 23mb to 13mb. And the quality remained the same. Thanks a lot. However, can you tell me how to change the ppi without changing the image size? Is this even possible? I wanted to change the ppi to exactly 300 before, but that always changed the size of the image from 50x70cm to 56x82cm.

First of all, you need to get used to the idea that Photoshop only works with pixels. At the core it has no concept of size.

Size is determined later with the pixels per inch number (ppi), which is really just an instruction to the printer how much to spread out those pixels. It means exactly what it says: how many pixels to how many inches.

Given the pixels in your file, the ppi number determines the print size. The bigger the size, the lower the ppi number, and vice versa. In other words, ppi is a way to translate from pixels to a physical size. It's just a formula.

If your image prints 50 x 70 cm at 338 ppi, that means the file is 6654 x 9315 pixels. That's the true size of your image. At, say, 240 ppi (which, incidentally, would be more than enough here), it prints at 70 x 98 cm. And so on.

OK, so far so good. Now for the jpeg file format. Jpeg uses data compression to reduce file size on disk and transfer. Jpeg compression is destructive, non-reversible and cumulative, so don't resave a jpeg if you can avoid it. It will degrade with every save. The reason it's still used despite this rather serious drawback, is that the compression is incredibly effective. It can shrink a file down to 2 - 5% of native size!, with very little immediately visible quality loss. Standard procedure is to balance the compression level to small size vs. acceptable quality.