Steadicam and similar systems are called "inertial stabilization" - inertia is the tendency for an object in motion to stay in motion. Inertia is directional - meaning that if you're moving the camera quickly straight down a hallway, it'll keep wanting to go that same direction and it will resist going in other directions. That "resistance" is what you're calling smoothness because it's resisting extraneous pans/tilts/rolls and translations in space. Now the amount of resistance to extraneous motion you get, the amount of "smoothing" you get is proportional to both the weight and speed of the rig. Some super basic physics -
M= mass (weight in this case)
V= Velocity (speed)
The reason it's important to understand that simple equation is because you know what you're up against in simple physical terms. Inertia = Weight multiplied by Speed. Which means that more weight = more inertia. Just as more speed = more inertia. More weight AND more speed = even more inertia. And with a steadicam or any other stabilizer that relies on inertial stabilization, more inertia = more "smoothness."
A very heavy rig moving quickly will be pretty easy to make smooth shots whereas a super light rig moving very slowly will be very difficult. If you want to get more into steadicam, light rigs are excellent practice because they teach you to use as little input as possible to control the rig. The whole purpose of a steadicam is to isolate all your "bad" movements as much as possible from the camera while allowing fine control of it still. The reason some brands (tiffin, pro, mkv, etc) are so expensive is because while the physics are simple, machining parts to tolerances tight enough to truly isolate movement without friction/stiction is difficult and time consuming = expensive.
Long-winded response but back to the OP question - more inertia or less over-control from your inputs. Inertia makes the rig want to behave so your own inputs need to be tamed (practice).