I have a 2 megapixel camera that saves its images in jpg format at around 400kB each. However, when I open it for editing for example in photoshop, it shows up as being 1.5 mB if I open it and simply resave it. Why is this so?

are you resaving it as a JPEG? probably because the image is being compressed a lot by the camera, PS has a great degree of choice here but defaults are for a much better quality compression than used in the camera.

I was confused about this once until I realized that PHOTOSHOP was saving the new JPG with my EPSON color profile. You can avoid this by unchecking that option when resaving. This may or may not be your solution but it is a cause for increased file size.

yes, I am resaving as jpeg. I dont recall using any profiles... just defaults. I noticed in the save for web, the original shows up as 5.49M! The filesize is actually 400kB. However, I can save it at medium quality without any noticeable difference for around 200kB