Not sure why the bag size is an objection. Sure it's not the size of a backpack (which is unrealistic) but it was never said to be small. It's just smaller than most bike backpacks (assuming they exist). The point is the bike is quite light so toting it won't be a massive pain after a short while. Why would you care if the backpack is a bit larger than a big pack? As long as it doesn't snag on every pedestrian you pass... you might have a problem in a massively crowded place... just carry an open can of paint in each hand and you'll have bucketloads of room.
Just think with its shape and size, perhaps a backpack isn't the best choice. A shoulder strap or messenger type bag may decreased the ninja turtle look a bit.