When I forward emails with JPEG attachments inline, the JPEG files size enlarge. Is this normal? Can I turn it off?
Yes. E-mail standards require that emails consists only of text; JPEG files are binary data. The inline images must first be converted into text, which increases their size. These are converted back into binary JPEG data at the recipient's end.
The Base64 Wikipedia article has more technical information and examples if you are interested in one of the most common ways this conversion takes place -- this one increases sizes by approximately 33%.