If the source material is large, it may not be possible to get it to a single layer without compromising quality. On average, you can fit between 60 and 90 minutes of reasonably decent quality MPEG content, encoded at between 4000 and 5000 Kbps, on a DVD-5, about 60% more than that if you use DVD-9. But compression rate will always vary based on the type of content, how busy the background is, how much movement, etc.

So it seems more likely that your software is having to overcompress the content, or perhaps there is a default bitrate that's too low. It doesn't make any sense that it would do well burning 2 layer discs but not single layer. I'd check your settings and see if you can find something there.

Other than that, we use Encore DVD, which is spendy, but very flexible and a good tool for assembling and burning DVDs and disc images.