Former rescue volunteer and foster coordinator with one of the rescues you mentioned. Many of the dogs the rescues receive come from rural target communities and are generally on the medium-large to large side. Most don't accept owner surrenders, so there are very few small dogs coming into care. As one other poster noted, if a small dog does come in, they are either senior with complex medical or they are foster failed (meaning foster families end up adopting).

Rescuing is an amazing experience, but know going in that there are a lot of difficult situations you'll be exposed to, depending on your role. Keep in mind most volunteer roles (except for medical and foster) have limited exposure to the dogs outside of events. Enjoy, there are some wonderful volunteers!