FHN, some of those women are definitely using supplements with testosterone in them. I don't think the average woman will ever look "too muscly" no matter how much she works out. A few women are addicted to working out, which is a condition that is usually tied to a body image problem and to anorexia, but the average woman can do all the pushups and situps she can and she won't look manly. The main thing that reveals the ab muscles is low body fat, not situps, and generally only starving oneself or running marathons burns enough fat to cause the abs to become distinct like that, in the absence of heavy-duty supplements.
Kacy Catanzaro managed to recently perform an absolutely incredible feat of strength and endurance, and I'd say she looks fantastic: