Works of art in three dimensions have mass. Some of these works also have implied weight as well. If an object is in three dimensions it also has volume. However, in the arts the question becomes, What is the function of that volume in that work? In most cases the answer is none. But, in most architectural works since the function of that structure is often dependent on the space inside, volume is a significant aspect to be considered.
Imagine running a foot race in a classroom. Now imagine running that race inside Ford Field (or some other enclosed stadium). The difference is the amount of space (volume) that is available for that activity.
Note that there are some sculptures that incorporate interior space in a similar way - even though it does not have the same functional consideration.