Some QBs (Brees included) get a chance to put up huge numbers because their defense is really bad. Brees does a great job, but he also gets the chance to pass the ball in a wide open offense for 4 quarters practically every game. A QB such as Aikman often had a big lead in the second half because their defense was so good. The Cowboys had a solid running game, so they didn't have the need to pass the ball. If you put Brees on that team, he wouldn't have the passing yards because the team didn't need them. If you put Aikman on the Saints teams of the last few years, he would have thrown for more yards than he did with the Cowboys because the situation (lack of defense and less of a running game) would dictate that he needs to pass for more yards. I enjoy watching Brees play, but his career has not surpassed Aikman's career yet.

Al Bundy wrote: Some QBs (Brees included) get a chance to put up huge numbers because their defense is really bad. Brees does a great job, but he also gets the chance to pass the ball in a wide open offense for 4 quarters practically every game. A QB such as Aikman often had a big lead in the second half because their defense was so good. The Cowboys had a solid running game, so they didn't have the need to pass the ball. If you put Brees on that team, he wouldn't have the passing yards because the team didn't need them. If you put Aikman on the Saints teams of the last few years, he would have thrown for more yards than he did with the Cowboys because the situation (lack of defense and less of a running game) would dictate that he needs to pass for more yards. I enjoy watching Brees play, but his career has not surpassed Aikman's career yet.

One thing I noticed over time with doing the research is that there tends to be a spectrum of sorts that involves a fairly basic formula: passing yards / passing TDs

On the lowest end of the spectrum (highest number) are those who play for poor teams that will get down, thus forcing a QB to pass for an obscene number of yards (since the defense will usually drop into a lot of quarters or a cover-7/8 to prevent the deep pass). The cutoff for this area is around 170 yards/touchdown and up.

On the highest end of the spectrum (lowest number) are those who play for teams that tend to be more run-oriented, with the quarterback's primary job being to turn and hand the ball off or run some play action. These teams tend to be substantially better. The cutoff here is around 110 yards/touchdown and down. This is also where you'll occasionally find a guy putting up a historic season.

The "normals" tend to cluster in the middle.

This year, for example....
Kyle Boller - 300
Marc Bulger - 293
Chad Henne - 240
Trent Edwards - 195
Derek Anderson - 296
David Garrard - 240
Kerry Collins - 204
JaMarcus Russell - 429
Matt Cassel - 183

and toward the other way...
Peyton Manning - 136
Brett Favre - 127
Drew Brees - 129