The truth is that the transition between playing formats has not been kind in every possible way. DVD provides a finite, somewhat small, amount of space for you to build on. For those who like to spend elaborate amounts of time with authoring software like DVD Architect or DVD Studio Pro this can be troublesome. There are a few ways to deal with the very limited space that conventional DVDs leave you with.
The most important thing to look at is the video compression you are using. You want to make an estimation of exactly how much video you need to place on your disc and then compare it with the highest possible quality you can get. Most compressing programs, especially those like Apple’s Compressor, will give you time labeled codec options that are preferable for specific video lengths. You may want to even go smaller, which means selecting a codec preset that is for a video length longer then the one you have. This will make the final video even smaller on the disc, allowing you to put more features and media on the disc menus The MPEG-2 presets for the longer length are going to accommodate for a longer possible film by shrinking it even more, which can be an option for further compression. In general, you are going to want to actually prepare the compression specifically for DVD authoring rather than actually trying to just take an existing QuickTime and then run into the authoring process.
Another way to do this is to design you own menus. Though menu templates are great, they are often very elaborate. If you can put together your own in Paint or Photoshop then you will end up with a smaller file. This will also limit your ability to put video clips on the menu, though if they are simply playing back assets that are used in the regular DVD playback then you will not need to actually utilize additional video content. You will need to simulate the disc before authoring to double check because anytime you try to reduce the menu file size you are going to risk disconnected links.
If you are going to use templates you need to limit what you do in them also. If you want to put video clips of the video that is about to be played you may not want to put the whole thing. Try just using a snippet, or even just cut together a bit of all the video clips inside DVD Studio Pro or the equivalent to use.
Try limiting the number of special features you end up putting on the disc. New authors often pack on a load of useless special features, like trailers for short films or dozens of boring production photos. Get rid of any that are not a significant piece of work on its own. This is not to say that you should avoid the idea of special features, and by and large this is a requirement of DVD authoring almost exclusively. This is even going to be true for film festival submissions as those special features can often be biographical and give important production and producer information. What is best is to determine exactly how many special features and what is necessary. On professional DVDs you often see DVDs loaded with these features as a way to legitimize the purchase of the commodity, but in your situation it has to be boiled down to what is absolutely necessary. Trailer, commentary, extra interviews, possibly a deleted scene or two, and text content is acceptable.
The last choice is to upgrade your disc type. Look into Blu-ray authoring or dual layer discs, both of which will up your possible volume. This was expected to be the absolute standard for us as we moved forward, but by and large Blu-ray disc has been ignored as a standard by the consumer public. It will often be acceptable for institutional and academic viewing, film festivals, and project delivery, but for home video production it is not going to have a universal playback like regular DVDs will. On the other hand, if you forgo the DVD authoring project entirely and move to a digital playback file you may actually have even more luck for larger files. It may, however, be difficult to inject the DVD menu and special feature formats into this.