This is an interesting topic.  The point is that yes, the balance point
and travel can be the same for short keys and long keys but there is
another important difference between the 2 that is significant.  It is
the actual mass of the key.  The longer the key, the heavier it is.
This means that more energy has to be applied to get it moving with the
same speed of a lighter key.  It also means that when it hits the end
stop, it has more energy to transmit to the next component.  This does
indeed change the characteristics of the touch.

Try making two identical travel levers from a short and a long piece
of wood.  Feel how different they are when you push them down quickly.
The same is also true for short keys with a lot of lead in them.
I think it is called the dynamic touch of a piano.

I think that pianos with long keys have another advantage because there
is enough space to balance the keys properly, less lead is needed in
them.  This could then result in the overall key mass being less than
on a small piano with lots of lead.

My personal experience is that pianos with long keys and less lead play
nicer than those with short ones.  Nicer being defined as not so trying
on the fingers and also more responsive somehow.  Player pianos are
among my favorites as far as touch goes.

Another observation is that the longer the key is, the more parallel
its front and rear move up and down.  Perhaps this also 'feels' nicer
to the pianist.  I don't know, I have played on a lot of different
pianos and the large ones with long keys are just best for me.  I am
sure someone else can be a bit more scientific than me on this but I
think I have the basics covered.

Kind regards,

Bernt Damm
Cape Town