Think what you're doing in the flare. The plane should be in a steady condition with a constant airspeed somewhat above the stall speed and constant vertical speed. What you're doing is turning that excess speed above the stall into lift to arrest the descent. You're trying to reach stall, zero vertical speed and the runway all at the same time. With a higher gross weight, the plane will have more inertia coming down. So it's going to take more energy to stop the descent.

The other thing to think about is your CG. The elevator will feel different at forward and aft CGs. With an aft CG it won't take as much force to rotate the nose.