Touchweight is really a catch-all sort of term, as far as what all goes into it (from what I understand, not being a tech). How loud a piano can play has to do with a lot of things - not all of which can be described just with "touchweight". A lighter touch piano might be more likely to be able to play loud, or less, depending on a lot of things. For instance (and forgive me here - this is all based on common sense, not any terribly specific technical knowledge, so please correct me where I might be wrong) a lighter "touch" can be due to lighter parts all around, say, and this could either make it easier to move them with enough speed and force to create great volume, or it could go too far and make them "wimpy" enough to not be able to transfer the power you're trying to put into them. Or, a lighter touch could mean all the same parts are there, but maybe there's less friction and less resistance - this would seem to suggest that this instrumnet could produce just as great a volume with less effort.
Many different scenarios I could see with many moving parts. And things that have nothing to do with touchweight could have a lot to do with volume, too - like tension. Soundboard health.
Pianos are complex animals and it seems nearly impossible try to pin down a piano's capabilities based on touch weight.
That being said, though.... I will say that I personally prefer pianos with light touch as they seem to offer much more capabilities in terms of the quieter end of things (when regulated well- greater nuance, greater subtlety which can be oh so beautiful. I'e played historic instruments which offer truly AMAZING capabilities in terms of this and opened up certain music in ways I never even dreamed possible before. Impossible to describe, really, but truly amazing and VERY informative. I'd recommend to anyone the experience of being in the presence of such an instrument played well (not just listening to the plinky recordings often found on youtube or such)- or better yet - securing the priviledge of playing one yourself.
I could go on for ages. I'll refrain.

INteresting topic, though!
JH