DVD, CD-Rom etc. Some are specifically designed for the low data rates on the internet eg. Quicktime's Sorenson codec and Realvideo. If your audience has to wait ten minutes and your movie sucks they will hate you. The frame rate can drop to about 15 frames per second and still be acceptable.
Detailed images result in a larger file size.
By reducing sharpness slightly and decreasing the contrast the file will compress better. After Effects and Media Cleaner can be used to blur less vital areas of the image to decrease the file size. With so many other movies vying for the attention of your audience you have to deliver quickly. Choose your shots specifically for the internet - Wide shots can leave the audience guessing what is going on. Subtlety is lost.