You can call the Big 12 what you want, but outside of Texas and OU, it is not "big time" football. To me, big time football is defined by how many butts in the seats the home teams have for games, the number of championships those teams have won over the years, how rabid their fan base is, and national recognition/following. The teams outside of Texas and OU have none of that. Sure the Big 12 kicks ass when Texas and OU are up, but who picks up the slack when they are down? In the SEC, if Alabama and LSU are down, Auburn, Tennessee are up. When those teams are down, Florida and Arkansas are up. Same with the Big10 and Pac12. I would put the ACC in the same boat as the Big12.
Average attendance per home game 2012 (I think a good benchmark for defining "big time football"):
ACC 48,519
Big 10: 70,387
Big 12: 58,712
Pac12: 53,586
SEC: 75,216
Striving to be in a conference the equal of the PAC12 and ACC is great if you're Baylor or Kansas, but it doesn't cut it if you are Texas with a 102,000 seat stadium.