The 3 strikes law was designed to end the careers of career criminals. That's why it matters that this time they stole a doughnut. So what, he stole a doughnut. It's a pattern of criminality that means he cannot be rehabilitated. That's why they are sentenced to life. Large crimes, small crimes, it's the concept that they commit crimes both large and small.
Every time a person is arrested for a crime they've been caught for, they have committed a dozen or more crimes but haven't been caught. That's why Al Capone died at Alcatraz after serving only 4 1/2 years. He failed to file income taxes for four years and allegedly owed $215,080.48 in back taxes. The government's case against Capone was so weak that 18 counts against him was dismissed before it ever got to trial.
Criminals are incarcerated on whatever they can be incarcerated for.