Why are keys in the lower register heavier than the higher register?

[rachmaninoff_forever]

Is there any particular reason why the lowest A is heavier than the highest C?

[pianowolfi]

Yes, there is a bigger hammer to move.

[rachmaninoff_forever]

So the size of the hammer determines how high or low the note is?

[pianowolfi]

Wait. What are we talking about now? The pitch or the key weight?
The keys in the lower register are "heavier" to depress (well actually they have just more inertia, as far as I know) because they have to move a heavier hammer. The pitch is of course determined by the length and tension of the strings.

[ted]

This seems to be much more noticeable on some pianos than others. I conjecture it is one reason few players seem to have quite as strong and independent finger striking in their left hands. Being perverse, I rather like the sound of rapid finger work, double notes and that sort of stuff in the bass, but it probably does require more practice to get the equivalent aural effect and control lower down.

[rachmaninoff_forever]

Wait. What are we talking about now? The pitch or the key weight?
The keys in the lower register are "heavier" to depress (well actually they have just more inertia, as far as I know) because they have to move a heavier hammer. The pitch is of course determined by the length and tension of the strings.

Well then if it's the length and tension of the strings, then why are the hammers heavier?

[quantum]

Greater diameter and length of the strings, require a greater mass to set that string in motion.  

Do some exploring yourself.  Take some percussion mallets of various sizes and try sounding strings with them.  Preferably you would do this on your own piano.  There really isn't much chance of hurting the piano, unless you start hitting things other than the strings like dampers or action parts. 

[hbofinger]

Bigger, heavier hammers. Also, if you compare with the way upper treble, dampers! (There are none in the upper). That only matters if you are not using the pedals!

I believe it is something that can be adjusted with weights in the keys though, if I am not mistaken.

[roseamelia]

In my opinion: I think that it's because you don't play the lower keys as much as the middle c keys and higher notes.

[robson]

Greater diameter and length of the strings, require a greater mass to set that string in motion.  

Do some exploring yourself.  Take some percussion mallets of various sizes and try sounding strings with them.  Preferably you would do this on your own piano.  There really isn't much chance of hurting the piano, unless you start hitting things other than the strings like dampers or action parts. 

exactly right, nothing really to think abut it,
basic principles on physics classes which OP obviously skipped out...