Well, most opera singers, with time and when singing with a healthy vocal technique, typically gain a 'larger' sound due to thickening of the vocal cords. Hence, for instance, a lyric baritone might be able, in his late 30's onwards (typically) to undertake larger roles (requiring a larger sound) in the lyric-baritone repertoire but this is only up to a point; a lyric-baritone will never be able to undertake dramatic-baritone roles (not with any degree of proficiency, that is). This also works in reverse; a dramatic-baritone that continously 'undersings' his voice by singing softer and in a lighter tone than comes naturally to him runs the risk of developing a wobble in the singing voice due to the strain on the vocal cords. I'm using baritones as an example here but this applies to all voices.
So yes, were one to actively work on this, the thickness of one's vocal cords could be increased but it is a long process (no 'quick fixes' here) and even then, works only up to a point. One would do better to work on projection and enunciation were they to want to 'increase' the 'size' of their speaking voice.
