Chicago Bears coach Lovie Smith has built the best defense in football by relying on a simple philosophy: Speed is more important than size. Chicago’s four starting defensive linemen – 302-pound right tackle Ian Scott, 300-pound left tackle Tommie Harris, 260-pound defensive end Adewale Ogunleye, and 262-pound defensive end Alex Brown – average 281 pounds, making them one of the smallest units in the league, but opposing offensive linemen haven’t blocked them very often this season.

At least until Sunday, when the Pittsburgh Steelers’ big offensive line (combined weight: 1,583 pounds) dominated the Bears’ defense. Pittsburgh finished the day with 190 rushing yards, controlling the ball for more than 37 minutes. Because the Steelers matched up so well with the Bears, it would be tempting to conclude that, at the end of the day, small defensive lines simply can’t stop the run. It would also be wrong.

Although some of the best run-stopping tackles in football – including San Diego’s 348-pound Jamal Williams, the Giants’ 325-pound Fred Robbins, and Pittsburgh’s 325-pound Casey Hampton – are enormous, an examination of the size of the defensive linemen throughout the league shows that size and ability to stop the run do not go hand in hand.

The basic statistical measurement of how well a defense stops the run is average yards allowed per rushing attempt. (The NFL ranks rushing defenses not by average but by total rushing yards allowed – a flawed statistic because it rewards good teams unfairly when they race out to early leads and force opponents to pass in order to catch up.) The defensive linemen on the top five teams in yards per rushing attempt weigh an average of 296 pounds, a mere two pounds heavier than the average weight of the defensive linemen on the five teams allowing the most yards per rushing attempt. The Bears rank fifth in the league, allowing 3.51 yards a carry, and their defensive line (small, at least, by NFL standards) weighs in at an average of 281 pounds.

Teams that use three defensive linemen and four linebackers generally have larger linemen than teams that use four defensive linemen and three linebackers, but even when 3-4 teams are separated from 4-3 teams, bigger lines are no better at stopping the run. The San Diego Chargers rank fourth in the league at 3.48 yards allowed per rushing attempt and have a huge 3-4 front of 303-pound end Luis Castillo, 348-pound Williams, and 309-pound end Igor Olshansky. But the Houston Texans are 30th in the league, giving up an average of 4.62 yards per run, even though they employ an even larger defensive line, with ends Gary Walker and Robaire Smith weighing 324 and 328 pounds, respectively, and tackle Seth Payne weighing 315.

The Pittsburgh offensive linemen who got the better of the Bears on Sunday weigh an average of 319 pounds, which puts them at the large end of the scale (NFL average is 310 pounds). The ease with which the Steelers ran against Chicago might hint that large offenses run well against small defenses, but the Steelers’ performance two weeks earlier showed they can’t always control the ball against small defenses. Against the Indianapolis Colts in Week 12, Steelers back Jerome Bettis had only nine yards on six carries, and the Steelers as a team combined for just 86 yards on the day. Indianapolis has a defensive line even smaller than Chicago’s.

Despite the lack of evidence that bigger defensive linemen are more effective against opposing rushing attacks, NFL general managers looking to shore up their run defenses increasingly draft huge defensive tackles. The early buzz indicates that the top defensive tackle prospects for the 2006 draft are Oregon’s 345-pound Haloti Ngata, Michigan’s 335-pound Gabe Watson, and Texas’s 330-pound Rodrique Wright.

Chicago’s biggest problem on defense against Pittsburgh was bad luck with injuries (both starting safeties missed the game) and with fumbles (the Bears forced the Steelers to fumble four times, but Pittsburgh recovered all of them). Fans don’t want to hear about bad luck, though, and if Chicago loses in the playoffs to another run-first team, some fans will say the Bears need to draft one of those players. Smith shouldn’t give in to that pressure. He’s built the best defense in the NFL for a reason: He knows that for defensive linemen, bigger isn’t necessarily better.

Mr. Smith is a regular writer for FootballOutsiders.com.