Whoever thinks it's short hasn't put a bit of thought into what happens at the end of the game. The perspective shift and rotation are hints.
Take a look at your total achievements for the game when you finish the 3rd boss. Hey, you have exactly half of them!
The others are the same, but mirrored horizontally!
That's a hint!
So if you start the game on the LEFT and pick the LEFTMOST first ball and take it to the centre to begin the game,
what would happen if, in a mirrored fashion, you move over to the _____ side and pick the _____most first ball and take it to the centre? Did you even explore the "main menu" (the initial room that you're in when you boot up the game)?
And believe me, you might be 50% through the game progress-wise, but you are less than 10% through gameplay-wise!
I love this game. I was so happy when, after the 3rd boss, I found it was far from over. Faarrr from over.