Why do some pianos have Keys that feel Heavy and some that feel Light?

The weight required to make a key go down is referred to as the "touch." On different pianos it might vary from 40 grams to 70 grams (26 grams make up one ounce). Differences in leverage, mass of various parts, and frictional resistance yield piano actions that play very differently.

But don't think that a lighter touch is always better. In fact, most advanced musicians like to feel a touch that is anywhere from 52 to 58 grams. If a piano is too light, there's no feedback from the piano back to the player. And if the touch is too heavy, arms and fingers tire easily and sensitive control is gone.

Some adjustments can be made to change a piano's touchweight. If you're interested, ask your piano technician.