Smirking to himself he slips little Kara into his jacket pocket to play with her later, though taking his hands off the superhero’s large, squishy bosom was a pain he had bigger things to worry about, such as finding out how she got like this.
Heading back down the stairs he wonders what could be able to shrink someone like Supergirl so easily, was it a supervillain, or just a random lucky alien. He's not sure but hopefully, it'll be a device or accident, though with her unconscious he's not going to learn from her anytime soon, all he could tell was that it must have had some kind of time delay, as she was in the stairway untouched, not something an instant effect shrinking would have allowed to happen.
Reaching the ground floor and then an alleyway just outside Catco, Carter spots a flick of red peeking out from under a box and walks over, pushing the box over to reveal what he expected: Supergirl's cape which as he lifted up, revealed a handful of what looked like porcupine quills, just with a fluorescent purple hue to them.
Curiously he gathers up the quills as well as folding up the cape, the only thing still normal-sized of his hero crush, and examines them, curious about where they could have come from, and it these are what shrunk Kara. It would explain why the cape is still normal-sized while they're inside it, she was pricked and infected, but she'd removed her cape and was heading back to work before they took effect.
What happens now?
1) While examining them Carter accidentally pricks his finger, shrinking to 1 1/2 feet tall due to being a human, short, but still more than enough to play with little Kara
2) While examining the quills Carter pricks his finger, but the alien's venom only affects females, so he doesn't shrink
3) While clearing up the evidence something brushed against this leg, it's another of the alien cat/porcupine creatures that shrunk Kara, though this one seems more friendly than the one Alex sent her to capture.
4) Just before he heads back inside Carter catches something out the corner of his eye, a shrunken Alex, having also gotten infected by the creature and coming to Catco to get help from Kara
5) Kara wakes up in Carter's pocket and panics, which only worsens when she discovers the alien's venom not only shrunk her but caused her powers to shut off