Can someone please explain why my JPEG attachments to an email are so large?

I have OS 9.1 with a G4 40 GB, 128MB ROM.  I take digital pictures using a
Fujifinepix camera and then download them using Exif Viewer.  I then process
and save them in either Tiff or JPEG format using PhotoShop 4.0LE. For
emailing I convert all files to JPEG and reduce them to what the computer
says is about 200-300 kB.  However, at this size, they have relatively poor
resolution considering the quality of the picture I take.

When I receive an email with an attached JPEG, the attachment typically has
a size of 100 kB.  When opened, these picture are usually of good quality
and resolution, typically better than the JPEGs I have saved at supposedly
300 kB.  When I save this emailed JPEG attachment to disk, and then try to
attach it to another email, the size increases from 100 to 300 kB.  Why?

Dr Eddy Wajon
16 Eckersley Heights
Winthrop  WA   6150

[EMAIL PROTECTED]