I would question the reality of that statement. Most of Santorum's supporters, so far as I've seen, are actually big-government "conservatives" who just want the big government to do the things *they* want it to do, instead of what the Left wants it to do.
That kind of person is not "very conservative," no matter how they may self-identify in a poll.
Do you have any evidence behind your statement? Given a choice of concrete evidence (exit polls), and your anecdotal evidence, I have to side with the concrete evidence.
And so far the concrete evidence is telling me that Santorum is the strongest conservative by quite a wide margin.
What do you see as the constitutional obligations of the federal government? Do you believe that the federal government should treat the unborn the same way as everyone else?
Why support Santorum if a person is a big-government supporter? Romney is far more progressive in terms of big government than Santorum. Most of Santorum supporters are strong social conservatives who don't trust Romney as far as they can throw him (and everyone else would be wise to adopt that thinking as well). Romney tries to reassure with his talk but social conservatives in Massachusetts know that Mitt Romney was their enemy when he was governor.