I compare the voice to a muscle - the more you use it in a certain way, the easier and more controllable it becomes when used in a particular range. Because I have spoken in a female voice for five years and trained in singing for one year, many people now think my voice has not broken because my body is so physically used to being used in a higher range. When I first started, I could barely articulate or sing notes beyond a G4 on the piano, but now I have increased my singable range up to a G5 (G above tenor high C) through vocal practice and directing the voice not outwards, but up and backwards in my head. This process is most easily described as if you do an impersonation of an opera singer, and you find higher tones are achieved more easily - although it shouldn't be too much of an impersonation, otherwise you get the notorious "Kermit the frog" singing voice.
With the higher range, more strength can be achieved by using head voice, as opposed to falsetto, which is the light, weak, airy tone that men (and women) end up using when getting to their higher ranges. Trained singers may still use it for stylistic effect.
But as Padma says, the more you use your whole vocal range, the more you can control all of it - and indeed a good singing voice is one where the entire vocal range can be used evenly and there are no abrupt changes in tone and quality, unless used intentionally.
Here is a video of me demonstrating my vocal range from my lowest to highest notes (the first five low notes are from pre-transition singing, the rest are post transition, hence the change in tone!) You will notice by Aflat4 my voice goes into the "mixed voice" range and then by Bflat4 into pure head voice.
Claudia Anderson 3 Octave Vocal Range F2-G5 (Contralto)
