It depends on a lot of different factors really.. would be a case by case basis. Depends on how good the transfer on previous DVD releases look.
A video can look great with a smaller filesize, as long as it is handled correctly.. but if the video is from the same source, it will always look better the larger the filesize, as less compression is required and also a higher resolution is possible
So its not as simple as saying it will be bad quality because each movie will be shrunk to a small size, but its nowhere near what bluray quality is considered to be. In this case, the bluray is just a storage medium which has a higher capacity than a DVD