Ok, my mistake. I thought that compression in iDVD was not to mpeg-2. Anyway, quality was not so bad. But I wanted something more, that's why i purchased Toast.
So, I created in Toast the same dvd which contains only one file (a quicktime movie .mp4 of 9GB). The video is taken mostly with an iphone, so I know that has nothing to do with high definition. But I wanted that titles, transitions, still images (with some ken burns effects) etc worked well. On the contrary checking the preview with the disk image, the result is pretty much worse than in iDVD. Nothing fluid and smooth: titles, images stuttering. Much more than in iDVD, that I consider not a great software.
A settings issue?
I customized the encoding using 7.5 mbps average max 8.5. I put a check mark on Half-pel. Did I forget anything?
And can you advice me: i have the same file in .mov (15GB). Maybe I have to start from this and not the .mp4.
Well, Thank you and sorry about my confusion (and my english, too. Hope everything is clear)