Effects of different bit rates?

[teamdork]

When encoding an .avi or .wmv, what effect does raising the bitrate from 128k to 20M have on the on the file other then making it larger and much better looking? I want to encode some short videos for playback from PC CD players and for progressive download from the web. The default template "bit rates" from my encoding programs (128k-3M) look bad, but when I turn them up to the max (20M) they look good. Is there any reason not to do this?
Thanks much for the help! Your comments are appriciated.

[redwudz]

The relationship is fairly simple: Higher bitrate=Better quality=Larger filesize. But you can go higher than needed and you may make them unreadable with no further gain in quality. A lot would depend on the quality of your source.

If you were using Divx or even the newer Xvid codecs, they have bitrate calculators with the codec adjustments. And if you look in 'Tools' to the left, there are bitrate calculators there. <<<<<<

If you plan on using them for downloading from the web, you may have to lower the bitrate and add some filtering to get them small enough for most downloaders to tolerate the download time.