At the time to make a movie the video Best quality size is 7.77 gb , after burn the DVD disk just use 1.97 Gb. , why this pinnacle file vs real burn size difference? any ideas or reasons?

Does your project consist mainly of photos?

music and photos 

Studio bases it's estimates on you using video content. Even with Pan and Zoom, you can compress photographs far more than video. As the program only uses one-pass encoding, it can't guess what the final files sizes are going to be with any degree of accuracy for slideshows. 

I didn't have this problem before, until now, there around 6 gb difference between, it is not normal, I hope I won't have this problem again,  I really appreciate your input, thanks

Ah, you didn't say it was a new issue, did you?

Is the quality significantly/noticeably lower than before? What duration is the disc?

Not much quality difference,  just that 1.99 gb size fit in an 4.7 gb but the over 7 gb forced me  to use a 8.5 gb, a lot of of space wasted, and it is not normal