Is 'Chlebuśka' an even smaller loaf? Or does that have different implications?
Not in nominative case. But if you really want to use it, you can. When there`s a will, there`s a way, too. :)
So, instead of saying chlebusia in accusative case: Chcesz pysznego chlebusia? Wanna yummy itsy bitsy bread? you could say: chlebuśka which is a variation of chlebusia. A possible invention but not as cute.