The large in-focus ant with robust mandibles is a worker of the Mediae cast. Mediae workers do most of the heavy lifting, cutting and carrying plant tissue great distances to the fungal garden. The smaller two ants (Minors) are on patrol, defending the larger Madiae worker from parasitic phorid flies. So no, these smaller ants aren't just getting a free ride. They are working too.
Learn how the some of the largest colonies on earth farm fungi and act as ecological black holes.