I have had a weak soft voice ever since I remember myself. Went through the frustrating experience of people constantly asking me to repeat myself because they couldn't hear what I was saying.
When asked to speak louder, I would try, but it would've been impossible for me to project my voice without shouting, and even shouting wasn't at a really high volume.
Always thought that the reason why that was happening is because my genetics or because I was nervous. Until I figured out how to improve my posture, and than I noticed some improvement in the volume and pitch of my voice.
I guess you can think of your voice as an acoustic musical instrument, at least it functions based on the same principles.
When it comes to acustic instruments, the shape is what determines the sound of it.
If you think at the shape of an acustic guitar it is not determined by esthetics, the body of the guitar needs to be a certain shape in order to amplify the sound correctly, the neck needs to be long and thick enough to stretch and maintain the tension in the strings.
Any deformations, could result in a change in the way it sounds.
There are three major components to your voice, the laringe, your vocal chords and your chest cavity.
The job of the laringe is to keep the vocal chords under tension, the job of the vocal chords is to vibrate and produce sound, and the job of your chest is to resonate and amplify the sound, since is mostly field with air.
Any structural changes to this components, could result in a change in how your voice sounds. For example when you were a child your voice sounded differently, as you grew up your chest got bigger, which made your voice louder and dipper.
Another time of life when most people are going to experience a change in voice, is when they get older. You know that stereotypical old person voice, high pitch, nasally, low volume, sounding like you always have something stuck in your throught.
This happens do to another structural change that affects the relationship between the three components of your voice, as most people advance in age, their posture gets worse and worse until it eventually changes the way their voice sounds.