Actually, the middle is not small, least of all in the Republican nomination process. Henry Olsen of the Ethics and Public Policy Center identifies “four faces of the Republican Party” — evangelicals, very conservative but secular voters, somewhat conservative voters, and moderates. He says the largest group, about 35 percent to 40 percent of the national party, are the somewhat conservatives. And in presidential years, moderates are the second-largest (25 percent to 30 percent). The somewhat conservatives “are found in similar proportions in every state” and “always back the winner.”
So if a candidate can sweep the moderates and also pick up a third of the conservatives and somewhat conservatives, that candidate can win the nomination.
It gets easier with time because voters flock to the front runner. It's not some conspiracy or evil magic.
All the same, though, I don't think Jeb will be the nominee. He's too tired and too hostile to conservatives.
That moderate group isn't what it once was either. A lot of them have gone over to the Democrats.