I suspect that many of the volume differences with similar quality instruments are related to a steel strung often being played with either a flat pick or finger picks which do tend to produce more volume than plain finger nails.

I find the 2 instruments very different and as I am flat pick orientated (and too stupid to play with fingers), tend to prefer steel strung more suitable for me - I'm no good as a player but I find higher tension strings easier for what I try to do.

I'll probably get corrected by one of the (many) more knowledgable guitar players than me here but that is the way I see it. I also see comparing a (good) classical to a (good) steel strung comparable to comparing a Rolls Royce to a Ferrari - both great but built for very different uses.

Jon