Playing pianissimo is a great way to evaluate a piano's action. When parts are correctly aligned, friction is reduced, and the movements of the key and hammer are properly adjusted, it's possible to play very softly with control.
And yes, it's much easier on a grand piano than on an upright: gravity keeps the various parts of the action in contact, while springs are used on an upright.
One key adjustment that affects "ppp" playing is how close the hammers get to the strings. Move the music desk on a grand so you can see the hammers, or lift the top of your upright. Push a key down as slowly as you possibly can, so that you don't actually make a sound. Just inch the hammer towards the string, without touching it. How close does it get before it falls away? It should be in the range of 1/8" to 3/16".
The farther the distance, the more "oomph" you have to give the hammer so that it carries to the string, and the less control you have over volume: sometimes it might be too hard, and sometimes not hard enough (making no sound).
These adjustments are called "regulating the action", and it's one of the three main areas of piano maintenance: tuning, regulation, and "voicing". For more info:
https://ptg.org/resources-pianoOwners.php--Cy--