It's not like any magic I've ever used, either. She can do some amazing things, though, like her teleportation and...

Oh, take a look at this! [ She takes the pouch and hands it up to Fionna, as she works on her legs. ] Doesn't look like it carries much, huh? [ And, indeed, the bag is small and seems to be completely empty from an outside perspective, or if she tries feeling it up. ]

Now... do you remember that sword you gave me? Reach in and think about pulling it out! [ And then be marveled at the Bag of Holding putting the hilt right in the girl's hand. ]