New York Times columnist Ross Douthat proposes the greatest explanation to why small-government conservatives are painted with the "extremist" label so recklessly by the left:
This divide, I think, explains a lot of the mutual incomprehension surrounding size-of-government debates. To liberals and many moderates, it often seems like the right gets what it wants in these arguments and then just gets more extreme, demanding cuts atop cuts, concessions atop concessions, deregulation upon deregulation, tax cuts upon tax cuts. But to many conservatives, the right has never come remotely close to getting what it actually wants, whether in the Reagan era or the Gingrich years or now the age of the Tea Party — because what it wants is an actually smaller government, as opposed to one that just grows somewhat more slowly than liberals and the left would like. And this goal only ends up getting labeled as “extreme” in our debates, conservatives lament, because the right has never succeeded in dislodging certain basic assumptions about government established by F.D.R. and L.B.J. — under which a slower rate of spending growth is a “draconian cut,” an era of “small government” is one which in which the state grows immensely in absolute terms but holds steady as a share of G.D.P., and a rich society can never get rich enough to need less welfare spending per capita than it did when it was considerably poorer.
There is more here.