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.
Santorum supporters just want the big government to step in and do what *they* want it to do, instead of what Obama's doing with it. Same motive, even if a different direction.