Not to be upstaged by Dora the Explorer, FIRESTAR has decided to obtain the biggest backpack by any means necesarry.
The fact that the majority of her vehicle mode is stored in this backpack does not bother FIRESTAR, because there is still plenty of stuff to go in it, and still plenty of room.
The fact that her backpack stretches to be twice as thick as her own torso does not bother her, because she can put her gun in there and everything.
The fact that she's clad up like an asian doll and wearing the highest of heels indicates that the giant backpack may be some new form of fashion. Perhaps we will all have backpacks the size of a full grown adult in the fetal position in the near future.
She has a big backpack.