Eldartford has part of it, the other part is amount of air moved by a certain area of driver. For low frequency if the room is too small then the driver will never be operating at it's optimum in terms of excursion and inertia at reasonable volume levels. The converse problem is also true if you try to use too small of a speaker in a large room and try to get realistic volume levels particularly in the low frequencies. The driver has to beyond it's typical limits and just can't fill the volume due to the small area moving air. All of the above is for dynamic speakers--planars have a different set of variables which I haven't really mentioned but many are in previous posts.
Why should some speakers need a big room?
It's often said that some speakers need a big room to perform well. Couldn't you just turn down the volume? And if the speakers don't sound nice at low levels, isn't this a fault with the speakers?
(And I never heard any say a pair of headphones needed a big head...)
(And I never heard any say a pair of headphones needed a big head...)