Many like to equate the size of the voice with its ability to be heard over an orchestra. The assumption is that a large voice will be heard more easily than a smaller voice. Of course, that depends on what is meant by a “large” voice but, based on the comments I have read, most consider a large voice to be one that has a broad or extremely full tone and some on this list consider a voice that can yell in tune to be even more acceptable. ...
If the soprano wishes to maintain or increase vocal intensity on an /a/ vowel in the range from about E5 and up, all-the-while keeping a vocal quality that closely matches her quality below these notes, she must gradually open her mouth as she ascends above E5. Acoustically she is tuning the first formant of her vocal tract to the fundamental and as the fundamental rises above E5 she must raise the first formant resonance to match the fundamental which she does by opening the mouth. Most female singers will do this instinctively. Middle voice notes below E5 will tune the second formant to one of the overtones of the fundamental. ...