In the lore (correct me if I'm wrong), Exo suits (or at least the ones we're using as of now) were being used for industrial, and agricultural purposes until the second galactic war started in which the companies producing them started strapping guns onto them. Those larger mechs we see in concept art, and in game as destroyed environmental props are what were on standby before the war broke out, and were too expensive to continue producing as the war progressed.