SFX power supplies are mostly used for in mATX systems. SFX PSUs, or variants thereof, have been used in systems by some big brands, including HP and eMachines.  But as the push to build smaller, quieter, more powerful system for appliance application (i.e. HTPC,  multi media PC to support Blu Ray).

This SEASONIC SS-300SFD Active PFC F3 - 80 PLUS Power Supply aims to provide that power but operate at 80% or better efficiency since most of these appliance system will be on 24/7/356.

All this but in the Dimension of: Length 4.9” x Width  3.9” x  Tall 3.0”