Having close to 40 years of experience of teaching and repairing voices, I accessed minimum to say over 20,000 people; some with talent, others without, some with good structure of the vocal anatomy and some did not actually have a physical structure suitable for any kind of singing, let alone professionally. Ultimately, the big mouth, the full lips, the good vocal box opening, would be desirable for anybody to become a singer of whichever calibre.
It's just like a classical ballet.
The person who would want to embark on a classical ballet scene, would have to have certain lengths of the arms and legs, certain flexibility, slender physique, and a high instep. The physical requirements for that particular field are very strict. However, outside of the classical ballet, there are some other forms of dance where those particular requirements for the physical structure are not as important.
A lot of components though, could be somewhat improved such as flexibility, body turnout and even the bad feet. So if the person has a real inclination, talent and musicality, she/he could still in spite of some body imperfections, could become a dancer and with the perfect technique could actually compensate for some of the physical body limitations.
Similarly, the person who is striving to become a singer may also have a musical talent, good ears, good pitch, excellent musicality, and beautiful expression of their soul. However, the physical anatomy might not be adequate enough to make it as a singer on a big scale.
It could be simply that the size of the mouth is not large enough, the vocal chords are shorter and thinner than they should be, and that, anatomically, might also be not corresponding with the actual size of the person. There is another variable, structurally speaking, which is very important.
It is the actual height (arc) of the vocal box itself.
Is it fixable? Yes, but certainly, with great difficulty.The problem is that, once I help the student (with the above described shortcoming) place the sound into the facial cavities, the sound often is not locking in, as the vocal box is flat and thus unable to hold the weight of the sound. Therefore, a lot of times the student and I could hear a crackling sound due to the voice slipping down to the vocal chords uncontrollably. However, with the great effort and dedication from both sides (mine and the student's) the mission gets to be accomplished and the musically talented person is not missing on their dream to become a singer, in spite of their defaulted anatomical structure. The vocal science technique, one more time again, prevailed. What about the person with the small mouth, skinny lips, smaller inside and outside opening, not to mention smaller and thinner vocal chords, you my reader may ask?
Could they become a singer?
The answer is yes, but a lot of technical work will be required to compensate for all of those imperfections. The technique has to become absolutely and totally precise, as they do not have much room for slack The sound has to have a precise height, a perfect width (body), and it has to be placed and totally locked in the set of the facial cavities. Any imprecision of the facial cavities working in conjunction and coordination with the abdominal and upper back muscles for the support of the sound, will cause the sound to be narrow, closed in a pear shape and to fall down in a lower position hitting the throat, vocal chords, larynx, and even the chest.
That said, the sound will never be able to fly over and above the physical body and thus will not be delivered to its aimed destination. So the question is; what would you chose, to possess a better and more suitable vocal anatomy or, the actual musical talent, the perfect heart and soul desire to express yourself via your feelings and emotions? Nevertheless, to have all of the above, would be ultimate.
But if any of the components are missed, you can still attempt to work on them to be able at the final stage to account for its totality, and then judge whether or not it is enough substance to achieve your goal and to which degree it will become realistic.