What could be the problem is that because iDVD can't create a dvd with a pixel ratio greater than 720 x 480. It doesn't matter if you play it on a blu ray player or HDTV, it's never going to be larger than 720x480. That's really tiny in comparison to even a 3.1 mp image which is somewhere around 2048 × 1536. You have a few options;
1) probably the cheapest. Hook your mac up to your HDTV and watch it that way. If you have a laptop it will be very easy.
2) blu ray burner and roxio's Toast program. To burn a blu ray disk that has a higher resolution
3) an apple TV or PS3 or Xbox, any other sort of set top box that can play high def content without needing a disk.
That's about it for your options to get a better picture. It really has nothing to do with how great the quality of the original image. It's a limitation of the device you're watching it on.