A typical grand piano action works against gravity: the hammers are underneath the strings and are propelled upwards by a series of levers connected to the keys. On an upright, however, each hammer is balanced upright and needs only be propelled sideways. Therefore, it's harder work physically to play a grand piano than an upright; this is compensated by a higher maximum volume with a concert grand because of how much larger the resonator is.

A baby grand isn't to be much louder (if louder at all) than an a full-size upright, but might still have less sympathetic vibration if the strings don't cross as much as they would on an upright. However, sympathetic vibration isn't necessarily undesirable; I'm convinced that the the opening bars of Schubert's Wanderer fantasy (D. 760) was composed deliberately to emphasise the effect.

Modern concert grands almost universally have the double-escapement action, which allows notes to be played very rapidly. That said, you only get the advantage of the massive leverage when starting from a state where the key is completely released, so all subsequent notes take a lot more force to be made at the same volume as the first. There are some uprights with double-escapement, but the are plenty that don't have it either... a 3/4-size 'spinet' a hundred years old is unlikely to possess such a refined mechanism (along with missing loads of strings, to add insult to injury).