There is often confusion, or questions, on why one Video file format yields larger, or smaller file sizes, than another one does. The answer is, on the surface, simple, but there are many variables, that make it a bit more difficult to give a definitive answer.
File size is governed by two factors, Duration of the Timeline (how long the "movie" is), and the Bit-Rate of the Encoding. A change in Bit-Rate directly affects the perceived quality of the file on playback, i.e. the "visual quality." That is the simple part.
However, one will be choosing the CODEC to Encode/compress the file. That CODEC choice will affect the visual quality of the output file, and also the resulting file size. As the Duration is set, the Bit-Rate is the only variable to affect file size. Some CODEC's will allow lower Bit-Rates (smaller file sizes), but at the sacrifice of quality. There are certain CODEC's, that allow for a lower Bit-Rate, but still provide adequate visual quality. A few, like DivX and its open source "cousin" Xvid, plus MKV, do a good job at compressing the file (lower file size), but still providing good quality. They are designed for streaming media, and the viewer must have them installed on their computer, on in their player, to play the resulting file. That can be a drawback, if one is say in a corporate office, where all add-ons must be approved by an IT department.
Another good format/CODEC for streaming video is WMV (for Windows computers, or Mac's with a utility, such as Flip4Mac installed), as WMV allows for good compression without too much visual quality loss.
H.264 is also quite good, and is a more X-platform choice. Again, the viewer will need to have the H.264 CODEC installed, but that is becoming almost universal. Some players, like Apple's QT Player (standard on a Mac, and easily installed for free on a PC) will install that CODEC. Also, there are several free players, like VLC Player and MediaPlayer Classic HC, that contain the H.264 CODEC, plus many more.
When choosing the format and CODEC for the Export/Share, there are many factors to consider:
- File Size
- Quality desired for playback
- Platform(s) to which the file is to be delivered
- Individual computers, on which the file will need to be played
- Availability of the CODEC for the viewer's computer
As you can see, it's now not quite so simple, and one must make some informed decisions, before just choosing any format, or CODEC. For the first two, file size and quality, think of them as being on either side of a balance scale. As the Bit-Rate goes higher, the file size goes up, and so does the quality. As the Bit-Rate goes down, so will quality, but also file size. Since this article addresses only files output for viewing, and not re-editing, only the editor can decide how far down the quality, the visual quality, can go, and still be acceptable. The choices are myriad, but the results are tied directly together.
For more details on CODEC's, see this article: http://forums.adobe.com/thread/546811?tstart=0
For more details on output formats/CODEC's, see this article: http://forums.adobe.com/thread/878590?tstart=0
Remember, we are talking about files being output for viewing, and not for re-editing. Some CODEC's that provide good visual quality at lower Bit-Rates, and the resulting smaller file sizes, like DivX/Xvid, MKV, WMV, etc., are NOT good choices, if additional editing will ever need to be done. For some viable options on files, that will be re-edited, see this article: http://forums.adobe.com/message/4556586#4556586