This is the answer by one of the developers about what Cuphead is:
We havent made ourselves too clear about things. Were in development, and we change our minds slightly about things as we playtest them. The majority of the game will be one or two screen bossfights, but we also want to incorporate platforming levels where youre actually running across eight, nine, maybe more screens. Itll still be a boss fight in a way.
For example, you might be chasing a boss. The boss could be in the background, and it would be a race instead of a direct Contra battle. Were trying to keep it pretty diverse even with that boss rush stigma the game has on it.
The single arena boss, just because its not as much work, is going to be a big chunk of the game. Well probably, just guessing right now based on what we have planned
30 to 35 percent of the game will not be that. Its 20 times more work, but (laughs), well do it.