You haven't been very clear Oak.

Links to these pages would be great. The file size of the 'image' remains the same, regardless of how or where it is displayed.

I'm assuming that you're after confusing your page file size with your image file size. It's possible that you are linking to the lightbox JavaScript externally meaning there is less code in your file.

When you mention 'px' or 'pixels' we are not talking about the file size. These are the dimensions, or how big the image will appear.

The file size is a different matter and is usually measured in 'kb' for images on the web. This basically determines how long it will take for the image to display or load on the screen and is completely different to the dimension of an imahe.

To have good quality images, you generally have to have over the 85 quality mark in Photoshop. This, of course will result in a larger file size than if you were to set it at the 20 quality mark.

Hope this helps and gives you a better understanding.

Ed.