I think the reason the main stage is so small is that not only does gaga not put into consideration what she has done in the past but rather sees the ARTPOP era as its own entity, and because the Born This Way castle did have problems with fitting within arenas in some parts of the world.
Can Gaga not do something different? It's a new album and a new tour, you have no idea what she has planned, so much is a secret of what she's doing. She isn't doing something "like Katy Perry" because Gaga is the most versatile and changing artist of this time so enjoy that Gaga is giving us this concert