One of the most important differences between different sizes of pianos is the length of the keys. They’re exactly like a see-saw, balanced on a fulcrum, and the weight of the action holds the back of the key down when it is at rest. After you press a key down, it has to return to its starting position (on an upright, generally) before you can play the note again. So “sticky keys” can be caused by lack of lubrication, a key rubbing on its neighbor, or problems in the action.
The shorter the key, the less it weighs. Also, short keys generally have to angle more at the back to reach the action at the breaks. This causes more side pressure on the bushings. Lastly, think about a key as a lever or pry bar. If you’re trying to turn a rusted bolt, you put an extension on your wrench to make it longer. Shorter keys give less leverage. And in side view, the shorter the key, the steeper they are when pushed down (the key travel at the end of the key is generally 1/2″ on all pianos). Take a look at this picture of a nice long key, on a full-sized (48″) upright:
White keys: overall length: 15 1/4″. Distance to fulcrum: 9 1/4″ to about 4″.
Black keys: Overall length: 13 1/2″. Distance to fulcrum: 8″ to about 4 3/4″.
OK, this is kind of dry, but wait til you compare it to a short key (picture soon). What motivated me to upgrade from an upright to a grand was learning the “Moonlight Sonata”. The first chord of the first movement in the right hand is G#, C#, E. When I played it, with the pedal down, it would sound like this: “La, da-da, di. La, da-da, di.” — the C# was striking twice every time I played it once!
Now, there are many possible causes for double-striking (including the piano needing regulation), but most uprights need firm pressure at the bottom of each keystroke to avoid this, especially with the pedal down. When you play that chord, you’ve got to have your finger all the way at the back of the key. This means you have the least amount of leverage. It’s even worse on spinet keys, which can be half this size.
Note that the balance rail pins are staggered to adjust for the shorter black keys. In fact, on this piano, if your finger is all the way in on a white key, you have about 3/4″ less leverage than on a black key!
More pictures soon.