The overall sound of a piano varies considerably with key velocity. Volume of a note is only one parameter that changes.
You also need to understand that a piano uses different string and hammer design at different parts of the keyboard. Lower register notes use wound single strings with dampers whereas upper register keys use three unwound detuned strings with no dampeners. Hammer hardness is also a factor in the 'sharpness' of the sound, which is why pianos often need voicing so that they remain spectrally consistent across all registers.
When thinking about sampling, you also need to consider both the spectral and sound content of the attack and the overall spectral envelope of the sustained note as a function of the key velocity. Basically, when you play a piano note with very little velocity, the character of the note will be somewhat different to that when played with a higher velocity.
The only way to really get a decent sampled piano sound is to sample an instrument across a number of velocity layers. It may be possible to sample and analyse the resulting sound for later synthesis, but in reality you might just as well use the original samples.
Pianos are certainly one of the more difficult instruments to sample correctly and well.