That big sound comes from its being one of Martin's vaunted "Vintage Series" instruments, so it has forward-shifted, scalloped bracing. As a slot-head, it has a 1-3/4 inch fingerboard (at the nut), which makes for easy fingering, too.
This large bodied instrument is in great shape, and is only $1999 withs its original, Geib-style hardshell case.
This 12-fret, slot head model has really great sound (I bought one off the store wall for personal use a few years back!).