You don't know that for sure. Hard to compare because you have the tired old argument if it's Big Ten offenses that suck so bad they make the defenses look good and vice versa with Big 12 defenses sucking
Last year Iowa played 7 teams with defenses ranked in the Top 25 of Football Outsiders’ opponent-adjusted defensive efficiency rankings. Iowa State only played 3 (with one of them being Iowa).
No guarantee that those defenses will be ranked as highly this season, but it’s pretty clear that Big Ten defenses are better than Big Twelve defenses.