JPEG is lossy compression so each time you save a new JPEG from an old JPEG, you're going to introduce further loss. A PSD is lossless and stores a variety of other elements in the file, which is basically why it's larger. Use a PSD to manipulate and then when you're done you can save out a JPEG or a 24-bit PNG file if you still want it to be lossless but smaller than the PSD.