A running joke on campus a few months ago was that Arizona running back Ka’Deem Carey’s head had gotten so big, he needed a new helmet.

After leading the nation in rushing, Carey allegedly got into a fight with his pregnant girlfriend over his smoking marijuana near her and then slammed her fingers into a door. Then, a few weeks later, he was ejected from the men’s basketball game against UCLA.

“Get the fuck out of my face,” Carey said to a University of Arizona Police Department officer. “Do you know who I am? I’m an All-American.”

Anyone who tries to use the old “Do you know who I am?” line makes it pretty clear they think they are above the rules. After being the toast of Tucson, Carey had developed a big head.

One of the other breakout stars of 2012, Texas A&M quarterback Johnny Manziel — also known as Johnny Football, the first freshman to win the Heisman Trophy — had a disastrous offseason, too.

Since winning the most coveted trophy, Manziel has allegedly shoved an assistant coach during practice, tweeted that he can’t wait to leave College Station, home of Texas A&M, been dismissed from the Manning passing camp, pled guilty to charges from a bar fight, and allegedly attended University of Texas fraternity parties and allegedly signed autographs for money.

The reactions and results Arizona and Texas A&M had to their troubled superstars couldn’t have been more different.

The Wildcats scrapped their “#teamkadeem” Heisman campaign, kept Carey from talking to the media for about eight months, suspended him for the season opener and then kept him on the bench for the start of the UNLV game.

Manziel was only suspended half a game ­— and not by Texas A&M, but by the NCAA.

When Manziel returned from his slap on the wrist, he embarrassed himself by making autograph signing and money gestures and getting flagged for unsportsmanlike conduct in the game against Rice on Aug. 31.

Quarterbacks are supposed to behave like leaders, not divas.

Then, after the sporting world spent a week attacking him, Manziel hovered over a Sam Houston State Bearkat, looking like he was taunting the FCS player.

Maybe Johnny Football was baited into his antics by Rice or Sam Houston State players, and Carey will surely face rougher treatment from opposing fanbases bigger than UNLV’s. But when the Rebels talked trash to Carey, they were bold enough to do so in the media.

Before the game, Rebel defensive end Jordan Sparkman said to the Las Vegas Review-Journal that the team would “dominate” Carey and hold him to zero yards.

Carey responded by rushing for 171 yards, including a 58-yard touchdown run on his first carry of the season. Carey did not celebrate by faking a smoke or slamming a door; all he did was lean forward when he reached the goal line, like a track sprinter.

“It hurt so bad,” Carey said about being suspended after the UNLV game. “It’s a humbling experience to see the team go out there and play. You just want to go out and be a part of it.”

In a one-on-one interview on Arizona football media day, Carey seemed genuinely remorseful and humble. Arizona head coach Rich Rodriguez said Carey had impressed him with his behavior since the basketball game, and apparently Carey has stayed out of Police Beat since.

Tough love looks to have worked.

In the book “Three and Out: Rich Rodriguez and the Michigan Wolverines in the Crucible of College Football,” Glenville State superstar receiver Chris George said that under Rodriguez, good players would not get a “hall pass” or preferential treatment.

A&M head coach Kevin Sumlin should have taken a page out of the book and just let Manziel run wild on the football field, not off it.

—Follow James Kelley @jameskelley520