Over the years, the shape of the classical guitar body has increased in size. There have also been other evolutionary changes
- changes in soundboard bracing
- changes in string tension
and so it is not a simple matter to attribute the sound qualities of the instrument simply to one factor alone.
However it can be said in general that the larger the body the greater the volume of the instrument but that beyond a certain volume the sound acquires a “boomy” quality – a lack of clarity and separation of the tones.
If you compare the modern flamenco and modern classical guitars you see that the flamenco instrument, while its length and width (of the figure of 8 shape) are very similar, its depth from front to back is less. The tonal difference between the two is largely a matter of more attack and slightly less sustain on the flamenca than on the classical.
(Attack – the time taken between plucking a string and the note achieving its maximum volume
Sustain – the duration of the note sounding after it has been plucked).
You also find that a number of electro-classical guitars have a shallower body. This is typically done so that feedback is reduced.
A question often asked is whether a cutaway in a nylon string guitar affects the sound. Its physical effect is to reduce the volume of air in the body and so given the above discussion it is likely that it reduces overall volume and sustain and increases attack. From experience I think these effects are there but not very pronounced and masked when comparing one instrument to the next by the natural variations in the acoustic properties of the different pieces of wood making up the soundboards.