noticed a lot of mention of how luggage can be a pain, especially on the subway & "rapid" (local?) trains. For two of us, we plan on doing entirely airplane carryon: Two small napsacks and two (overhead-compartment-friendly) rolling bags. Would this be considered "bulky"?
If it fits in the overhead bin of an airplane its probably going to fit in the overhead compartments of the Shinkansen. For local/commuter trains etc, it shouldn't be much of a problem unless you got onboard with those bags during the height rush hour where things are more jam packed, otherwise you should be fine.