Problems with DVD capacity

I've had this problem with every DVD I've tried to make with iDVD 6. The last DVD I made was 2.8 GB in size as reported in the Project Info. I encoded it into a disk image and the final size is 4.1 GB. Luckily I'll still be able to burn it to a disk. The problem arises if I try to make a DVD any longer than 3 GB. The encoded size is bigger than a DVD can hold. If I burn directly to a DVD instead of making a disk image, it will finish the encoding, then when it gets to the burning stage it will give me an error saying that there is not enough room on the DVD to burn. My settings are set to Best Quality single-layer DVD. I've had this happen on two different computers.
On a related note, the capacity bar on Project Info is either green, yellow, or red. Obviously when it's red, that means that it's over capacity. However, the bar is yellow from about 3 GB to 4.2 GB. What does yellow mean? If I ever encode a movie while in the yellow capacity region, the final size is greater than a 4.7 GB DVD. As long as I stay in the green region (<3GB), I'll be fine. Any ideas?