as much as i'd like to dismiss ohio, or at least the small-minded people of ohio who wouldn't vote for a black president, we might (and might not) need ohio because as one in-the-know person put it, in the general election, either the democrats get ohio's electoral votes or the republicans do.
in the general election, it isn't about the popular vote, or which candidate has the most votes when you tally the votes from each state, it's the electoral college votes of each state that decides. remember how al gore won the popular vote but what's- his-name stole the electoral college in 2000.
in the general election, most states have a winner take all system so if most of the people of that state vote for the republican, then the republican gets all the assigned electoral votes. so states such as pennsylvania, florida and ohio are states that are meaningful to each party because they don't reliably vote for one or the other as does california--democrat, new york--democrat, texas--republican.
california, texas, illinois and new york have the highest number of electoral votes. see this nifty electoral college map.
but clinton's "big states" argument, that she's better at winning the big states really doesn't matter when it comes to big states such as New York (31 electoral votes) and California (55 electoral votes) because, they are likely to vote democratic, regardless of whether its she or obama who is the nominee.