It is still weird, though. Do we have any bounds on the relation between size of the object and time you can maintain the transfiguration. Harry can maintain (without contact) some transfigurations for a pretty long time, even with a large-ish object like a cauldron. Unless the time you can maintain a transfiguration decreases very fast with the size of the end object (Harry transfigured a unicorn and a big rock into something small and maintained it easily), there is no reason why Quirrel would not be able to create transfigured clones able to clone themselves. And we know that transfiguring an inanimate object into something living can easily be done for strong wizards, as McGonnagle proved early in the story with the desk into pig thing.