You also have to take into account that there just were not the heavy tanks we have now back then. The largest was the steel 72 that has excellent buoyancy characteristics and many smaller tanks and sets. Double 50's, 72's, and triple 30's were common. With one reg, no back plates, and only canvas web straps for a harness they also did not have much extra stuff to compensate for. Now if you go to the warm water and dive in shorts and a T-shirt, with one 72 you are almost perfect for weight at all depths. Even with a 80 all you need is a 4# weight to be all set.
Other things are that the reg of a double hose regulator wants to be level with your lungs. That means between you shoulder blades which is one reason you may see old photos of divers with tanks way low on their backs with the tank bottom at mid butt level if not lower. This does two things, it makes you heavy behind so that you take a head up angle when swimming and this attitude helps you plane through the water. It also gets the mouth piece a little above the reg which helps with its performance.
It is all a learned, or unlearned now, diving style. BC's came out of the cavers when they took bleach bottles down and added a bit of air as needed and balance on them like a kid in a pool with a beach ball. Then Fenzy came out with the first real BC in the late 60's. I used a horse collar BC up through about 1984 or 5 when I got my first jacket BC, a SeaQuest ADV. This jacket was also the first BC I had that had a power inflator. When you had to blow up a BC by mouth, you just were a lot better at working a bit heavy or light.
