It is not a trick. It is normal. Most mainstream DVDs released today (fortunately) now use more of the available bitrates, which results in a noticably better picture quality especially for those viewing on larger displays. There are occasional exceptions, but the visual quality of most DVDs has increased dramatically in the past two to three years, and you have noticed one of the reasons for this.
Keep in mind those original discs are compressed already. And, the higher the compression rate, the more likely you will be able to see picture degradation (artifacting, macroblocking, pixel breakups, smearing, etc.) Four years ago, most discs released were full of this. Today, consumers have demanded a higher quality, and the industry had to respond. The (relative) success of Superbit DVDs showed Hollywood that there is an audience who cares quite a bit about picture quality. (Note -- it is not JUST Superbit discs which brought about these changes, but they were one factor... Hollywood noticed how many people were buying certain discs because the transfer was so good, or the restoration was was so good). For awhile, there, it seemed almost a backlash against the "loaded extras." Note how some studios are now experimenting with a 1 disc and 2 disc release, simultaneously. It satisfies those who demand high picture quality, and rakes in extra money for those who insist on the "extras."
Solutions -- Split. You can then keep 100% of the image quality. Or, use a dual layer blank disc. You can then keep 100% of the image quality. Or, go ahead and compress (with a program like DVDShrink) and gamble with the final result.