I am quiet when the conversation is not about a topic that I find particularly interesting. When the topic is interesting to me, I have lots of thoughts about it, and my default setting is to say these thoughts out loud, whether I am alone or with others, so I become loud. I am quietest when the conversation changes topics a lot. When I am having a more socially aware day, I might filter my thoughts a little, but once I really get into a conversation about an interesting topic, all filters disappear, and I am very loud, say a lot, and possibly drive other people nuts. In terms of volume, I also speak louder during these times, and I am expressive with facial expressions and gestures, all of which are meant to convey more information about the topic and have nothing to do with theory of mind, although they may be miscontrued as such by others.
So I can be very quiet or very loud, depending on the specific situation. My quietness is not due to anxiety or shyness, but it can look that way to others. My loudness is like a runaway tractor trailer driven by interest and being socially unaware.