From what we know Splatoon is the first game that is coming out from the Garage project (Miyamoto talked about it last year) and that Nintendo has greenlighted three more prototypes to become full games.
http://purenintendo.com/2014/09/03/nintendos-newest-development-process-labeled-garage/
I believe, as I am a software engineer/project manager myself, that all these game projects share between them the engine programming team, the tools programming team and the sound engineering team after the recent company restructuring Nintendo brought together all its development teams under the same building/area - and I think that the 40 people that made Splatoon are focused entirely in the game design and not in engine/tools/sound programming
Thanks for linking that article, I had never seen that. Very interesting indeed! Maybe we'll see another new IP at e3 from Garage.
So, between music composition/mixing/programming, the engineering team, and the core team of 40, we could assume the team is 60-80 people? That's not an entirely small team...and we havn't considered testing and club nintendo debug unit.