There are many moving parts in a piano action and depending on so many factors, they all influence the "weight" of the key. Key depth, single or double escapement action, the wippen, the repetition mechanism, how tight all these parts are such as the keybushings, friction, and there are even external factors which can affect it such as temperature and humidity. Also, some pianos such as the Steinway are prone to vertigris.
Ultimately, if ANY piano feels stiff to you, you are most likely playing improperly. First, it is the weight of the arm that depresses a key. If you try to play from the long flexor muscles, you will experience fatigue, cramps or sloppy technique. There are other muscles such as the pronator, supinator, and even the bicep plays a role. There are no muscles in the fingers so finger exercises and "building strength" is a myth. Technique is all in your head.
Remember being a teenager and going through a growth spurt and you tripped over the floor or fell walking ups stairs? That is because your brain needed time to figure out the new measurements of the body. Once your brain figured it out, you were able to walk again. Once the brain figures out the length of the fingers, the weight of the arm and other ancillary movements, they keys are effortless to depress. A single bad habit can stall technical growth.
What hampers pianists mostly is not necessarily not using the proper muscles but using the improper ones. Abducting, twisting the wrist, playing with a still or quiet hand, not playing in/out, not playing on the edge of the keys, not playing with circular or shaping motions, pressing into the keybed, not playing to the point of sound, playing from the fingers, etcetera.
If you would like to play something heavy, find an old Tracker Organ in a church and play that. Old trackers have mechanical actions where when you press a key you are actually moving a four foot piece of wood. Every pianist and organist should play one because it immediately exposes your every technical flaw. If you find the keys hard to press or you can't play evenly or you fatigue quickly . . . your teacher failed you.
Playing a Tracker