Photoshop :: Replace Burnt Out Windows

I am almost a begginer with PS7, and circumstances require me to use a feature asap before my instruction book arrives!

I make 360 panoramas, and I have a series of .jpg digital pictures taken inside, some of which contain 'burnt out' windows. I have however also taken an identical picture exposing correctly for the outside lighting.

I wish to replace the 'burnt out' windows with the properly exposed ones.

I have got as far as cropping the outside shot and placing it underneath the inside shot and using the eraser to make it show through. It seems to work quite well, but my problem is the saved file size. The originals are approx 1.5Mb, but after doing the above, the file size goes beyond double the original .. which I can't seem to fathom. I don't understand why this should be so, and when it will come to stitching the 12 images, they need to be roughly the same file size (and exact dimensions).