Those are very few exceptions to the rules of many decades in the life of the Kirov/Mariinsky Ballet. What seems to be happening today is a more prevalent pattern, and erring on the side of choosing too many for the mantle. Since both Nureyev and Sizova were from the same generation (born a year apart), I'm wondering if this happened to these two dancers because of directorship policy at the time or perhaps the overall political climate in the Soviet Union under Krushchev's government, affecting the political appointees at the theatre.
Corps work, and a number of years of corps work used to be mandatory for all in the Maryinsky.
Except for Rudolf Nureyev and Alla Sizova, who jumped from graduating from the Vaganova to principal immediately. At the Bolshoi, Maya Plisetskaya did the same.
In Imperial times and Soviet times, this was often the case as well. Displaying the future czar's jewels was as big a badge as the miltary medals worn by the husbands who had really good placement in the May Day parade. In the Soviet Union, not only was casting impacted, but also the ability to tour: only the reliable were allowed to go. (Nureyev again was the exception that proved the rule.)
The fact of the matter is that, very often, dancers like Somova are pushed into principal roles prematurely due to (perhaps IMO) political connections, sponsors, and such, and not necessarily due to talent.
How much is the company reliant upon touring? One of the pressures on artistic directors is to present the next new thing. I would think that the Mariinsky also has pressure to prove that the Vaganova School still produces star dancers and to show them off on these tours. I always wonder how often young, talented dancers are pushed as prodigies and "discoveries" to prove that a given institution or artistic director has it in him (alas there are few hers) to recognize the special ones. It doesn't have the same snap to advertise "The wonderful dancer in her mid-20's whose been carefully nurtured in the classical style and the nuances of presentation, and is now in bloom," as "18-year-old wunderkind jetes over the Oresund Bridge."
Regarding the corps in classical ballet, I think this may be the case of "you had to be there." It is such a different experience to see the corps of the Mariinsky or Paris Opera Ballet at their best, and how the corps dancers breathes life and is the core of these ballets. My analogy would be Casa Mila (La Pedrera) in Barcelona: static in photos, and organic and alive as a sea anemone in person.