When I look at some balance diagrams, I don't understand why the forward limit is higher with an airplane "heavy" than with a light air plane.
Hi,
It may be due to the maximum downward force the tail (elevator and stabiliser) can effect to balance the aircraft (with a more forward c of g) with Land Flap and at minimum approach speed.
Your minimum speeds will increase with increasing mass, therefore the force the tail is able to exert also increases.
See
https://safetyfirst.airbus.com/under...t-and-balance/
"As an example for take-off, if the CG position is too far forward, the aircraft has such a “heavy nose” that the correct take-off rotation rate using the elevator becomes impossible to reach. The impact of an excessively forward CG position on aircraft maneuverability applies at all phases of flight. However, it is most noticeable at low speed due to the reduced effectiveness of the elevators."