So here is a comparison of sizes between the GS mini and Martin 0 and 00's
Martin Size 0 (14 fret): Body Depth at deepest - 4 1/4" Body Width at widest - 13 1/2" Body Length - 18 3/8"
Martin Size 00 (14 fret): Body Depth at deepest - 4 1/8" Body Width at widest - 14 5/16" Body Length - 18 7/8"
Taylor GS mini: Body Depth at deepest - 4 7/16" Body Width at widest - 14 3/8" Body Length - 17 5/8"
Of course the scale length of the Martins is longer at 24.9", while the mini is 23.5", but I have a 00-18 Martin and it is nowhere near as loud as my mini mahogany, so it doesn't seem to suffer for that 1.4".
The late John Pearse, iconic acoustic guitar player and Martin aficionado believed the Martin size 0 to be the optimum size for the folk guitar as quoted on page 240 in the book, GUITARS by Tom and Mary Evans.
Source of Martin data: MARTIN GUITARS - A History by Mike Longworth
Source of Taylor data: Taylor Website