OK, mystery partially solved, but this still isn't right. It appears that including the metadata in the exported file--even if there isn't much metadata included with the sequence--causes the bloat. But a full megabyte's worth? That seems extreme.
Turning off the metadata inclusion helps somewhat, but the file is still larger than it should be. The video reports as 500kbps, the audio as 96kbps, but the total bitrate is 664kbps--that's not correct. I can export a smaller and better looking FLV (or MP4) from CS4 or from Squeeze, so what is CS5 doing to overstuff the export?
Did some more testing on this issue, this time with a 30-second DV clip. I thought maybe it was the sequence that was causing the bloat, but an export directly from a raw clip yields the same issue. I've tried exporting six ways from Sunday: from a sequence or from the Source Monitor, using either direct Export button or sending to the AME Queue, by adding it directly to AME, with metadata embedded and with it turned off. No matter what, the streams are reporting the correct bitrate, but the final muxed bitrate is far higher than it should be; I also tried encoding at a higher bitrate, and the same bloat occurs.
Can anyone take a few moments and confirm this behavior? MediaInfo using the Tree or Text view will show the stream sizes and their percentage to the total weight of the file. Also, I've done a little testing with H.264 exports, and the issue does not seem to affect raw clips. Sequences exported to H.264 are a different story, though--some degree of bloat happens there. It would appear that the streams are encoded correctly, but my guess is that somewhere in the muxing process, a bunch of extra padding is being thrown in there.
If this is a bug, would this be reported as a Premiere bug or an AME bug?