Encoding question: estimated size vs. final size

Using iDVD 7.1.2. I've been reading about encoding options (professional, best performance etc.) and still not sure I quite understand what is going on in this case.

I have a single movie, 74 minutes, in DV format in my 4:3 project. To eliminate extraneous stuff, I reduced the main (and only) menu to all text, with a plain theme with no motion.

I go to Project Info, select High Quality encoding, DVD type Single-Layer.

In the color bar area, the arrow points to the green, the total is 74 minutes, and it estimates 2.32GB.

I then try to burn the project using "Save as VIDEO_TS file" (or Save as Disc Image...). I keep the Project Info window open. _As soon as_ it starts encoding the video as seen in the progress window, the arrow in the Project Info jumps into the red, and now estimates 4.02 GB. Sure enough, the final folder/disc image is 4.02 GB.

So what is going on here?

a) Did it just badly estimate the final size by almost 100%? That doesn't seem like the right answer, since first of all, how did it not know beforehand, but the secondit started encoding, it suddenly knows for sure? Also, my understanding was that you could fit approximately 2 hours of video on a SL disc; I am well below that, but somehow already nearing disc capacity.

b) Did it realize that there was only 74 minutes of video on the disc, and just adjusted the level of compression/bit rate or whatever the technical thing is to a "higher quality" rate to utilize the full capacity of a disc? Would, say, a 90 minute or 100 minute or 110 minute video all result in a roughly 4 GB end result? This seems more plausible, except the "quality estimate" arrow in the Project Info then becomes misleading. It is now in the red, making me think I better encode with Professional Quality, when in fact the quality should be even _better_ since it gets to utilize the whole 4 GB for a project that should only take 2.32GB (or whatever).

This actually has practical significance for me. My real issue is that when I burn the disc, it starts flaking out near the end of the disc when I play it, as if I am just going too close "to the edge of capacity" and it is not stable. Now, maybe that also reflects badly on my disc media, or DVD player, or something else, but let's assume I can't really change those things. How do I improve on this? If (a) is correct, I can improve this by trimming material, to get a point where I can encode it at Best Quality to get a real capacity of 3.9 GB roughly. If (b) is correct, then trimming material will do nothing, since it will just "expand" the encoded file to fit the available space. What I would need in that case is a 4th "compress to fit into ___ GB" option, but that is not an option. In that case, I need to trim enough material so I can get it to fit within the Best Performance encoding option, which is CBR and thus presumedly the final file size is directly and predictably proportional to the video duration.

Thanks for any insights.