Dwarf Mondo is Ophiopogon japonicus. It should reach close to a foot in height, with ~15" leaves. There are, however, cultivars that are more dwarf. A common nursery offering is 'Nana' which only reaches 4-5" tall. It's a neat and trim little plant, especially suited to rockeries and alpine arrangements, or containers.

It sounds like you got a dwarf variety. Black mondo grass in general is not as vigorous or fast growing as the regular green varieties -- which I guess explains why it is so expensive. I have both the "tall" and dwarf varieties of regular green mondo grass. The tall variety grows much faster than the dwarf, but the dwarf does fill in quicker once it becomes established.

There is no recognized form of "dwarf" black mondo - Ophiopogon planiscapus 'Nigrescens' - in fact, there is in reality only one true form with various cultivar names ('Arabicus', 'Ebony Knight', etc.) generally considered synonymous with Nigrescens. Foliage height can be very variable and a lot will depend on growing conditions. This plant is not a true OG but a member of the lily family and prefers an acidic and rich, well-draining but moisture retentive soil and at least part sun for best growth. And it is indeed slow to establish and spread.