The "problem" lies in the compression. JPEG is a very good compression model for photos, but it is destructive. If you save the file as a Photoshop file, you use a nondestructive compression which is not as good in compression ration but does not alter the image each time you save it.
And then, when adding layers, each layer you add is like a picture on it's own (with a mask, which is also a picture on its own). With a lot of layers, images tend to grow quite big. There is no "solution" to that.
You could use, however, the TIFF format with ZIP compression. That format keeps the layers (if you want) and uses one of the best nondestructive compression algorithms. The drawback is that saving takes more time.
ABAMBO | Hard- and Software Engineer | Photographer