RENTON — The little things are supposed to make all the difference.

Not for the Seahawks' defense. At least not this season. Size has been everything when it comes to measuring Seattle's improvement.

"One of the most significant things that changed is big plays," coach Pete Carroll said.

A year ago, the Seahawks allowed 76 plays of 20 yards or more, tied for second most in the NFL according to The Associated Press. This season, Seattle has given up just 45 such plays, tied for second fewest.

That explains why the Seahawks have one of the most improved defenses in the league. But it will face a stiff test Sunday in Arizona.

"They've had a lot of explosive plays," defensive coordinator Gus Bradley said of the Cardinals.

Arizona had seven plays gain 20 yards or more in its last game alone, and it has five plays of 40 yards or more in the past four weeks.

Those are the kind of plays that Seattle has been so much better at stopping this season, though it hasn't been perfect.

No one has forgotten Michael Crabtree's 41-yard catch to set up what turned out to be the 49ers' game-winning field goal last Sunday, or the 50-yard touchdown pass the Seahawks gave up to Washington's Rex Grossman in the fourth quarter of the Nov. 27 loss.

But the Seahawks are a lot better than they were a year ago, something that speaks to the quality of play from cornerbacks Richard Sherman and Brandon Browner, who have proved capable of playing man-to-man, press coverage. And more than anything, it speaks to the improvement that free safety Earl Thomas has made in his second year.

"He's been so much more efficient and effective and on the right spot so many more times," Carroll said.

That's a big reason that not only was Thomas voted a Pro Bowl starter at age 22, but the Seahawks are allowing 19.5 points this season, down from 25.4 a year ago. It is the fourth-largest improvement in the league this season (see chart). The Seahawks are allowing 32.2 fewer passing yards this season, the fifth-largest improvement in the league.

The Seahawks have ranked No. 25 in the league in points allowed for each of the previous three seasons. They currently rank fifth.

"The chemistry is good together," linebacker Leroy Hill said. "The athletic ability of each guy is so different, but it fits so well in the defense. The parts that they finally put together, and running this defense, is just perfect."

Notes

• WR Ben Obomanu (knee) and LB David Hawthorne (knee) both returned to practice Friday, and are listed as probable for Sunday's game.

• T Breno Giacomini (abdominal injury) and DE Anthony Hargrove (calf) are the only Seahawks whose status is questionable for Sunday. Both practiced Friday. Giacomini sat out Thursday's workout because of the injury, but is expected to be available. If he's not, expect Jarriel King to play at right tackle.

Danny O'Neil: 206-464-2364 or doneil@seattletimes.com. On Twitter @dannyoneil