This was an unfortunate trend I noticed in parts of Sinderella too, it's not always strictly a volume thing either, some of her arrangements are just so busy it overpowers her vocals. It's not something that detracts too much which is why I still like the music, but it is something I've noticed.
Sinderella's bigger problem for me was the stylistic pitch correction though - I get what they're going for, but Calli's vocal skills feel wasted on overproduced pop music.