As someone who is very non-talkative, it's usually a question of mental energy but context is important.
When the topic at hand is something I don't care about, and therefore have little value to add to the topic, I stay quiet out of politeness. In the wrong kind of situations this can be frequent enough that folks just think I'm timid or a jerk. It's really a harmony judgment on my part though, a form of Fe call I think.
Whenever I am tired or stressed then I conserve all mental energy I can, that will inevitably show as quietness.
In most ordinary instances where I am not stressed and tired, I may talk more but my own style is to talk when I know I will be heard, since to do otherwise is a simple waste of effort on my part. This can show me as someone easily "talked over" by more boisterous folks and that happens sometimes. It's another example of politeness on my part, and folks who monopolize the proverbial floor irritate me quite a bit
One side effect of this is I have become quite trigger-happy about "zoning out" when others steal the floor, and this is habitual enough that sometimes I ignore information or even hesitate to interject when it's my turn.