In reality, the actual perceived loudness will depend on the distance between source and listener. MIR Pro tries to mimic this natural phenomenon by means of two features, namely "Distance Dependent Scaling" and "Simulate Air Absorption". Both of them affect the dry signal component only (... as the headline "DRY SIGNAL SETTINGS" in MIR's GUI implies :-) ...), but of course they will change the overall volume of the instrument in a mix.
Natural Volume is meant to be used as a starting point for a "healthy balance" within the virtual orchestra. If you're unsure about it you can always switch off both features, but then you will have do all the distance-dependent dry/wet balancing manually. .... I tend to call this the "pop mix mode", because it makes it easier to keep things under control when realism is _not_ what I'm after. ;-)
HTH,