It is just simple physics.
A "heavy" ball is ball with a lot inertial energy due to its pace and spin. Spin holds a lot of inertia, it is very difficult to impart directional change when there is a lot of inertia. So, when you hit a ball with a lot of spin, it feels "heavy".
A "fast" ball with a light spin is going to feel different because it may not have as much spin on it and thus not as much inertia. If you are hitting it stright back where it came from, it is going to feel heavier, but if you are hitting it an oblique angle, it is going to feel lighter since you are not taking on the directional momentum head-on.