It's a bit of a self-fulfilling statistic. If you beat a team, they have less of a chance of ending the season with a winning record. If you lose, they have more of a chance of ending the season with a winning record. So by default, games against teams that are 8-7 in the rest of the season become "big games" ONLY if you lose them. If you win them, well, it wasn't a "big game."
Take the Arizona game last season. If we had won, Arizona would have finished 8-8, and it wouldn't qualify for the list of "big games." But we lost, and they finished 9-7, so it becomes a "big game."
We split against the 9-7 Commanders in 2007, so they both are classified as "big games." If we had swept them, neither game would be classified as a "big game." We split with the Eagles in 2007, too. They finished 8-8, so neither game is on the list. But if we had lost instead of beating them 38-17, both games would have been on the list. We beat the 8-8 Vikings that year, and it doesn't qualify for the list. But it would if we had lost.
In 2006, our victory over the 8-8 Giants doesn't count, but it would if we had lost. Our victory over 8-8 Carolina doesn't count, but it would if we had lost.
Plus, a meaningless Week 17 game in 2007 is included as a "big game." Not to mention that games can retroactively change to being a "big game" or not being a "big game." (If the Giants go in the tank, Sunday's loss will not have come in a big game. Just like last year's win over the Packers, who were 2-0 at the time, doesn't qualify for the list even though it was a big game at the time.)
If we take away the meaningless Week 17 game in 2007 and include any game against a team that finished with a winning record against everyone else (at least 8-7 if we played them once or 8-6 if we played them twice), Romo is actually 9-9 against winning teams, not 7-10. And against teams that won at least 10 games against the rest of the league, Romo is 5-4.
WINS (with opponent and their regular-season record against the rest of the NFL)
06 Panthers (8-7)
06 Colts (12-3)
07 Giants (10-4)
07 Giants (10-4)
07 Vikings (8-7)
07 Commanders (8-6)
07 Packers (13-2)
08 Eagles (8-5-1)
08 Giants (12-3)
LOSSES
06 Saints (9-6)
06 Eagles (8-6)
06 Seahawks (9-7)
07 Patriots (15-0)
07 Giants (10-4)
08 Cardinals (8-7)
08 Steelers (11-4)
08 Ravens (10-5)
08 Eagles (8-5-1)