Video bitrates and keyframes

This is barely tangential, but I’d still like to ask the question. I’m gearing up a clog (that’s a cartoon blog) that serves a series of cartoon videos via the Brightcove platform. I have Flash 8 which includes the ON2 VP6 encoder. I can set the keyframes at any frequency I want, from a keyframe every frame to letting the program choose a wide spacing.

Here’s the question: When I set the frequency of keyframes close together the size of the FLV file increases. Which makes sense since there is more data to be stored. But how is it that I can increase the size of the file and still hold the delivery data rate constant? I encode the video at 500kbps to match the Brightcove standard. If the entire file has to be eventually delivered, how come two files of widely different sizes (because of the different keyframe spacing) are delivered at the same bitrate?

In short, I’m trying to understand the downside of including more keyframes, versus holding the bitrate constant.

Thanks,

TW