no, i don't think the hit box is bigger, but it can feel like that at times is what i'm saying, when you are on your twelfth try on the boss, and the have two pixels overlapping with the trail and instantly die, it doesn't feel difficult, it feels annoying, and frankly poorly done, and the rest of the game feels like a perfectly crafted, where you can always do something better, or catch back up if you fall behind, the boss gives so little flexibility that it feels like it was ripped out of a different game.
(Edit: also, sometimes the trail and the boss isn't affected by invincibility)
(sry about the paragraph, I do love this game, and just want to give as much feed back as possible)