Olennas Yeh, I think curvy is a stretch, especially in professional companies. I do think that's sad, I also think it should be very different in children's classes, and recreational classes. Russian training is still pretty brutal, they take children very young and do work them to extremes.
The Royal Ballet School are much much better, although they do still have a 'look' they want for their own company, which is to do with their current dancers (other women in the corps, men who will partner, costume sizes etc...) dance companies are not wealthy, and they do reuse costumes and shoes and stuff, so they basically want dancers that will fit, rather than having to alter things too much or buy new things.
There will also be the need for dancers to do the steps, modern choreography is a hell of a lot harder than the old stuff, audiences want crazy high legs, and insane lifts, and slightly alien levels of bendiness. Non-traditional companies, who take dancers of different body types, or who have more men, have less people going to see them. That sucks too. But it all contributes, skinny people weigh less, they can be lifted higher, short people can wear pointe shoes without changing their centre of gravity too much or being too tall to partner, slim legs look longer and show a cleaner line, which is what people 'expect.
These expectations are wrong, but they are real. They are NOT real in kids classes or recreational dance schools, then they are 'enforced' by shitty teachers who probably have a superiority complex and imagine they are training future professionals who need to look the part. Largely, they are not, any kid good enough will be in an associates program heading for a pre-pro school at 11 or 16.