Guitars were much smaller a century ago than most guitars being made today. To get a bigger sound, E.F. Martin developed the "square-shouldered" "dreadnought" style. Rather than recontouring their guitars, Gibson simply expanded the contours, making them bigger. "Jumbo" guitars, sometimes called "Super-jumbo" guitars were favorites of string band leaders, singing cowboys, and others. To this day, there are folks who prefer the sound of a jumbo to the sound of a dreadnought.
The RiverboatMusic.com "buyer's" guide tells the history and rationale of these huge instruments, and provide some tips for shopping for a "jumbo" today. http://riverboatmusic.com/guitar/jumbo/jumbo_guitar.htm