This issue always seems to confuse folks, so thanks for asking, Shahid. There are a variety of values associated with photo size/resolution/quality, and they are only somewhat related to each other.

Let's start with megapixels. That's the key spec by which most cameras are sold, and it refers to the sheer number of pixels in the photos. A 1-megapixel camera takes photos with a million pixels in them; a 20-megapixel camera captures 20 million pixel photos. Think of it like a grid that tells you how wide and tall the photo is. For example, my 16-megapixel camera takes photos that measure 4928 by 3264 pixels. Multiple those numbers and you get 16 million pixels. Note that this says absolutely nothing about the actual quality of the photo--just the size.

Then there's the file size. A photo might be 16MB, 2MB, or even just 100KB in size. This measures how much space the photo takes up on your computer's hard drive. There's no direct relationship between file size and megapixels, because file size also depends upon the amount of compression used to save the photo. A TIFF or a RAW photo generally has no compression, so all 16-megapixel RAW photos are always exactly the same size. But if you look at a hundred 16-megapixel JPGs, you'll find that no two are the same size. The file size depends upon the compression used to save the photo and the amount of detail and color in the image to begin with. So the file size can give you some hints about the quality of the photo, but I wouldn't try to read too much into that, either.

You mentioned that even seemingly small photos look excellent on your computer monitor. That's not surprising. No matter how many millions of pixels are in a photo, it'll always be resized to just 1280 by 1024 on your monitor, or however many pixels your display is set to.

As you can see, measuring image size and quality is difficult. And I've overlooked the most important consideration: taking a great photo to begin with. The photo needs to be focused sharply and properly exposed. All the megapixels in the world can't fix a bad photo.