KNOXVILLE, Tenn. (AP) — When it comes to estimating the weight of mammoth Tennessee defensive tackle Daniel McCullers, numbers are a relative term.

Tennessee coach Butch Jones said McCullers is "a very, very slim ... 360-plus." McCullers contends he's actually down to 352 pounds, the lowest he says he's weighed since early in high school.

As he adjusts to a new coaching staff and a switch to a 4-3 defense, McCullers undoubtedly has slimmed down. But he's still carrying quite a burden. Tennessee's chances of improving a defense that statistically ranked among the worst in school history last year depend largely on McCullers' progress.

"We'll be as good as Dan McCullers goes," Jones said. "It is what it is."

McCullers doesn't mind hearing that message. He's accustomed to it.

"I kind of knew it was coming," McCullers said. "All along, since the spring, he was always on me, just to make me better."

The Volunteers allowed the most points (35.7) and yards (471.4) per game of any Southeastern Conference team last season. Tennessee scored 36.2 points per game a year ago, but the Vols finished 5-7 because they couldn't stop anyone.

Tennessee's offense may take a step back this season as the Vols attempt to replace quarterback Tyler Bray as well as first-round draft pick Cordarrelle Patterson and second-round selection Justin Hunter in the receiving corps. So the defense will have to be much better.

"It starts with stopping the run," Jones said. "It starts up front. That's where it begins. Obviously he's going to be a big part of that. If we can get him to develop and improve, that's going to lend to us being a better defense."

McCullers had 39 overall tackles and 5 ½ stops behind the scrimmage as the starting nose guard in Tennessee's 3-4 defense last year after transferring from Georgia Military College. McCullers called it nothing more than an "OK" season that "wasn't what I want to be."

This year, the Vols need McCullers to do more than clog the running lanes. They want him to make plenty of big plays himself.

"It's time for me to step up to the plate," McCullers said. "The defense is depending on me. That's what (Jones) said. So I'm going to give it all I've got."

That means not stepping up to the dinner plate quite as often. McCullers, who is 6-foot-8, needed to lose weight to have the quickness necessary for his new role. McCullers says he wants to get down to "340-ish" by the start of the season.

In last season's media guide, McCullers was listed as 377 pounds. McCullers said he weighed up to 395 pounds while playing in junior college, though his teammates believe he's been even heavier than that.

Senior defensive lineman Maurice Couch hosted McCullers on a recruiting visit to Tennessee and said McCullers weighed "like 420" pounds at the time. Couch remembers wondering at the time how someone so heavy would contribute to Tennessee's defense.

"I was like there's no way he's going to play on the defensive side," Couch said. "He was just real massive. I felt they were going to make him (an offensive) tackle or offensive guard or something. He proved a lot of people wrong."

McCullers believes he still has more to prove.

In a year when four Tennessee underclassmen bypassed their senior seasons to enter the draft, McCullers opted to stay in school. Preliminary forecasts of next year's draft suggest McCullers could get taken in the first two rounds.

McCullers said the decision to return for his senior year wasn't particularly difficult.

"I knew I had to come back," McCullers said. "I feel like I had a lot more learning to do to get better."

Every day is another lesson for McCullers as he adjusts to a new body, a new scheme and a new staff.

McCullers disputed the notion that he'd won over his coaches with the progress he made during the offseason. He knows he still has more work to do. The same can be said for the entire defense.

"I haven't won them over at all," McCullers said. "We've all got a long process ahead of us. They're going to continue to be on me because they know I'm a big player in their defense. I have to make big plays."