ABC wrote:
I notice this with the files I work on. After saving, the file gets
much bigger, although it has the same format.

So I opened a .jpg file of 677k size, did nothing to it and
immediately "save as" in . jpg with a different name. My Photoimpact
7 opened a dialog box and I chose 100% quality so I should have no
loss. The new file is now 1.7M in size but with the same image( in
fact there seemed to be some slight change in the quality)

why is this? how can I get a file of the same size and quality ? Is it
just Photoimpact??

JPEG compresses images in a way which means some data is lost - the amount
of data loss is varied by the quality setting. If you chose no compression
then it will be maximum size. What has happened in your situation is that
you've opened a 667k file, which decompresses to about 2MB. When you then
save the image again you have to re-compress the image again to get it back
to 667k - which would require further loss of image data to get it back to
that size. If you chose no compression (or actually, no data loss
compression) then you end up with a 1.7MB file.

This is exactly why people use RAW/lossless image formats, and only save as
JPEG when outputting the final image. Each saving of a jpeg will lose more
and more data (unless no loss is specified, in which case, you get
no/practically no compression).

David