@Ceili,
I'm not sure exactly when they decide "we need more reps". Every million people? I dunno, and that seems wrong.
They do move the #s around, though, and states can (and do) lose seats due to populations shifting. NY used to be the second-biggest in terms of #s, now it's third (Texas is #2 these days).
And yeah, candidates don't go everywhere, and they likely never have. It's expensive and it's time-consuming, plus these people are on 24/7 and are trying to preserve their health (amid a boatload of jet lag, among other issues) during a very long haul.
The NY Times noted that that was one of Romney's issues. He was behind on $$ (yes, even with that huge war chest; he lost a lot during the GOP primary fight) and so he was going to fundraisers when he really needed to be out there pressing the flesh.
Plus, also in the "bang for your buck" end of things, I gotta figure candidates mainly spend time in cities or at least somewhere near an airport (that used to be train stations, of course). So for a state like NY, it's likely that time would be spent in NYC, Buffalo, etc., assuming the state was in play (it wasn't, and it hasn't been for some time, but let's just posit that as a condition). But it's unlikely that a candidate would haul ass all the way out to Montauk Point on Long Island. It's nice there, but it's remote from everything, there's no airport and there ain't a lotta people. Epic fail if you're short on time, funds and need to get outta Dodge quickly.