Re-sizing issue

Jpeg uses very aggressive data compression to reduce file size.

A file format is a storage format. It only applies when the file is packaged for storage and transfer. When the file is open in Photoshop, it is decompressed, and when it's open inside Photoshop it doesn't have a file format. There, it's just an array of pixels with certain properties.

Be aware that jpeg compression is destructive and cumulative, and every time you resave a jpeg it deteriorates a little. When you save as jpeg you never get exactly the same file back.

What you mean in Photoshop? Are you looking in status bar or Info panel? 

So you are saving file as PSD or some other file format? What is file size of saved document on your drive? There is huge gap between Photoshop and email which you must fill to give us idea where to look for problem or what can be wrong. And lastly, what is actual problem? Content is not as expected or simply you are wondering about where is lost weight?

Update: I eat info about file type probably because you can not create jpeg in photoshop but rather file which can be saved or exported as... many file types.