IIRC this was associated to an early, brief spike in atmospheric oxygen. It went up, fell, and then took about one gigayear to increase again to large sustained levels closer to what we have today. Large organisms have low area to volume ratios, and require a high level of oxygen to scale up while producing enough energy. See https://ars.els-cdn.com/content/image/1-s2.0-S16749871120008...

Source: https://www.sciencedirect.com/science/article/pii/S167498711...