Bitmap graphics are made up of tiny little dots called pixels. Two things determine the number of pixels in a bitmap image: resolution, or pixels per inch (PPI). An image that is 72 pixels and has a resolution of 72 and is 1"x 1" contains 72 rows across 72 pixels each: it's 72x72. Using all your fingers and toes, that comes out to 5,184. That is a lot pixels for such a small image; not to mention that the pixels contain information about each color. Vector based images do not contain all this extra bulk, making the image file sizes much smaller.
Another problem you might run across in your graphics venture with bitmaps is resampling (changing the pixel size). Consider the 1x1 image--a one inch square. If you resize it to 2 inches, you are still limited to the original pixel information. Even with the multitude of resizing options, which help smooth out the jaggies (a staircase of lines that do not fit perfectly in the grid). The software is simply not able to create data that wasn't there to begin with.