How come digital photos from the same lens/camera take up different amounts of memory?

I have a Canon Rebel XT camera. Some photos (Jpegs) only take up about 1.2MB while others take up 6MP. Why is there variation when the same camera/lens is taken for each photo? Also, Canon XT is an 8mp Camera. Shouldn't each photo be 8mp? Thanks!!!

PS: Photoshop tends to increase the memory of a photo. Why is that?

First off, megapixels aren't the size of a photo. It's what makes up the photo. You are thinking of megabytes, which would be the size of a photo and how much room it takes up on your memory card.

Yes, more megapixels usually means more detail and a larger size, but that's not always true. And depending upond your settings (Fine, Normal, etc) can change the size and quality of your photo. Also, EXIF data is enclosed with each photo you take, and not every photo has the same details. Every picture is going to be a different size. Just depends on the quality, etc.

1. The number of colors has an effect, also the amount of different objects. A closeup photo usually takes less than a long shot with many objects and many more colors.

2 Photoshop adds information to the photo file and that makes the size larger.

Digital SLR's like the Canon Rebel XT allow the user to select the size and quality of the images. The highest quality jpg will be a large file and take up more memory of your CF card. The lower quality jpg settings compress the image more which allows more images to be stored but the quality is compromised. You may ask, why would anyone want to take pictures that are lower quality, why not shoot everything at the highest quality? Well, those who shoot only for web publishing and some print don't need the higher quality so they go for the speed and ease of the smaller images.

In addition to jpg, your Rebel XT offers a RAW setting that creates the largest and highest quality image. The RAW setting requires post processing. The files are huge and take up a great deal of memory and take a long time to buffer. I suggest you shoot at highest quality jpg unless there is a specific reason for having smaller files.

The Rebel XT is an 8mp camera. The imaging sensor contains 8 million pixels. The more pixels a camera has, the higher the quality that is possible with the camera.

The XT is a wonderful camera, I shoot most of my paid work with one and it matches the quality of cameras costing much more.

If you are shooting RAW unprocessed and uncompressed photos the file sizes are identical and depend on the number of megapixels in the camera.

If you are shooting jpegs where the camera compressed the image to remove what it's firmware thinks is unnecessary detail file size depend on the amount of detail in the photo.

A few experiments

Take a picture of blue sky with no detail--very small file size.
Take identical photographs of something with a lot of detail at all your ISO setting--the file size with increase at the higher ISO setting because the additional noise is 'unwanted' detail.

And. you can make big differences in file size by changing the amount of jpeg compression in the camera menu but I don't think that is your question