I agree with what you’re saying. It amazes me when I hear a 1930s Martin D-18. The guitars are light, but extremely powerful and easy to see why Doc Watson, Kenny Smith and many other’s use them for recording. There’s an interesting posting on UMGF by a guy who is comparing his guitars. See: http://theunofficialmartinguitarforum.yuku.com/topic/137995/1936-D18-vs-2011-D18-Authentic
He weighed his D-18 Authentic 1937 (made in 2011) vs his actual 1936 Martin D-18. There was a 5 oz weight difference between them. 5 oz!!! The older being lighter, yet it has the more fuller sound.
I assume it’s the shrinking of the glue, the drying of the wood etc. This comes only with time.
Given a choice, however, I'll take a late 40s-early 50s Gibson J-45 or Southern Jumbo (the same guitar, but with a Grand Ole Opry pimp factor). It has more of an 'Americana Sound' and you feel the low notes in your gut.
One thing that helps the sound is that the nitrocellulose lacquer finish becomes harder and thinner with age. The wood also ages more for some years, and I think all the vibrations simply loosen the guitar so it can vibrate more. All I know is, it makes a difference, and I think it makes more of a difference with mahogany soundwood than maple or rosewood. That's just my opinion.