Most of the DVD's I do have chapters does that make a difference? How where you able to get DVDA to render when it says 6GB?

Chapters in themselves won't make much difference. However, the more menu pages you have for scene selection the more space will be required.

When DVDA tells you the size is too big it's only a warning. Ignore it. You can click the Prepare and Burn buttons anyway.

Is it not true that the size of the file after compression depends on the content of the video? For example a compress one hour video of a black screen would compress much smaller than a detailed video with lots of changes frame to frame ... like a constant pan?

That can make some difference, but not as much as you might think. Any image or motion at all will be encoded up to the bitrate specified when you render. Even a still image would tend towards the the specified bit rate rather than fewer bits since each I-frame would be a complete new copy of the image and the encoder will use as many bits as it can to faithfully reproduce the image and maintain it until the next I frame.* It would take an awful lot of solid color to drop substantially below that rate. Since an hour long video of solid black would be pretty boring and not very commercially viable, it's unlikely you'll encounter this situation often.

* What happens during action is not that the encoder uses a different amount of bits at these times**, but that it would require more bits to maintain the image quality. Since it can't have more***, it must let the quality suffer. Conversely, when the motion is low or nonexistant, the encoder will use those bits for more quality.

** Well, true for CBR encoding. But, even for VBR the amount of spare bits available when the action changes from slow to fast isn't inexhaustible. VBR can't handle long passages of fast action any better than CBR.

*** It is, afterall, called a "bitrate". This means that, with very few exceptions, the rate of bits being used at any given time tends to match the specified bitrate. This is also why the size of the encoded file depends almost entirely on the bitrate and the duration, while factors such as frame size, frame rate, and content make very little difference to the file size.

**** Erratum, there is no fourth footnote. Or, well, then again, maybe there is, which would make the notification of error itself in error, which would mean that the appropriate word would then be Errata, except that it would then no longer be in error, and then should therefore be Erratum after all ...

First, are you compressing to MPEG-2 in Vegas or Architect?
Whatever the case, just try lowering the bitrate a little bit!
Second, just like the other guy here says, it could that your other video is more rich in content and/or has more action in it, which could be influencing your final bitrate and file size.

I can't believe how often this comes up, and we all come up with the same recommendations.

I believe that you will find the answer has to do with how you created your DVD. Normally, if you reference the same media from more than one place in your project, DVDA will only use one instance of that media, and your project will not grow. Very smart.

However, there are very subtle things you can do within a chapter to cause DVDA to create a brand new VOB file from the same media, thus dramatically increasing the data burned to the disc. For instance:

If you add a subtitle to one instance of your media, and then have another refernece to the same media without the subtitle, DVDA will duplicate that media, thus increasing project size.

The same thing applies to multiple audio tracks.

There are many other situations. Rather than repeat what I've posted before, let me provide links:

DVDA estimate problems

DVD problem: Here's the solution!