perhaps it is not the size of the lungs but the quality of the aveoli in them. the smaller lungs could be better suited for breathing because munch is a water critter and would need to hold his breath for longer periods of time. they may need less "air" for more breath then a glukk does.
-or-
perhaps she has many gabbit lungs in her and thats why they needed to hunt them for medical purposes. to learn how to make a better lung for their queen. they could fit tons of those where her lungs should be.
-or-
her lungs are not inside her body (the giant pipe shes hooked up to)
-or- her lungs are inside her mouth
there is all sorts of creative ideas on this that i can come up with. i say just pick your favorite explanation (except for discontinuity
and work with it. they will tell us if it ever really becomes a major problem.