But they can still breed offspring that could breed right?
Provided the chihuahua has access to a ladder.
Precisely. Barriers to reproduction come in two primary forms:
1) Prezygotic, which in itself has two subcategories: pre-mating and post-mating.
2) Postzygotic (intrinsic, extrinsic, etc.).
Any (or either) one of these is sufficient to establish reproductive isolation, and one need not be accompanied by another in order to consider two individuals separate species from one another.
Specifically, the barrier we're dealing with here is mechanical isolation: a prezygotic barrier to reproduction which functionally prevents mating between two individuals (so a pre-mating barrier) based upon some sort of anatomical incompatibility. Without human intervention, it seems extremely unlikely that these two breeds would be able to mate with one another by any conventional means. Depending upon the context, this could well be sufficient cause to consider these two breeds separate species.
There are also potential postzygotic barriers here. For instance, it may be that only male-Chihuahua/female-GD pairings will develop to term, given the size difference between the two. I don't know whether this is actually the case, however.
Human intervention muddies the waters here, of course, facilitating an otherwise impossible (note: this is an assumption on my part) reproductive event between two lineages.