Spacious, commodious, capacious, ample are comparable when they mean larger in extent than the average.
Spacious implies great length and breadth and, sometimes, height; primarily, it is applied to things that have bounds or walls.
In its extended use, though it usually implies limits, it suggests largeness, sweep, and freedom within those limits.
Commodious stresses roominess and freedom from hampering constriction along with convenience and comfortableness.
Capacious stresses the ability to hold, contain, and, sometimes, receive or retain, more than the ordinary thing of its kind.
Ample basically means more than adequate or sufficient (as in size, expanse, or amount). It may suggest fullness and bulk and in extended use it often suggests freedom to expand or absence of trammels or limitations.