Ryan gave you a perfectly good explanation. DVD has limits and that is 9.8 mb/s (video) and 10.08 payload rate (video/audio combined).
You need to think of it in time, not file size. Thats why bitrates are offered in mb per SEC. He also explained why you should not go near the bitrate ceiling. 7-8mb/s max for burned media.
This is why you need to do a bit budget every time you make a disc. What the length of your program? Thats the first question….from there you would use a bitrate calculator (or some some simple math) to find your optimal bitrate based on your project. Here’s a calculator
https://www.videohelp.com/calc.htm
I find it hard to believe Apple training never mentioned bit budgeting.