His force is about equal to Bolton's, and both armies have suffered from the snows, though Stannis is probably worse off. However, what I got from the last Reek/Theon chapter is that Stannis was blowing the horn to attack constantly, or at least frequently, in order to keep the people in Winterfell on edge. First they'll lose sleep while expecting an attack, then they'll stop worrying about the constant horns, and then Stannis will actually attack, and they'll be unprepared since they think it's just another feint. And by then they'll probably be weakened by the heightened vigilance of the past weeks, so Stannis would be able to stand a fighting chance.

Furthermore, Bolton's plan, and the reason he is so confident of victory, is that the Karstarks are ready to betray Stannis. But the last time we saw Asha, Theon had finally come to the camp. And he's one of the very few people that knows about the Karstark betrayal, and I'm sure Stannis will be made aware of that.

Though on the other hand, wasn't Roose planning to march out of Winterfell the last time we saw Theon, before his jump from the wall? Considering the Asha chapter where she sees Theon comes several weeks later (from what I understood) and there was no mention of there being a battle, it's entirely possible that Roose headed off in the wrong direction with his force, and might even be lost or dead now. Since Stannis found a fairly decent place to sit tight while the storm rages, while Roose would have no shelter at all. That could also explain why it is Ramsay and not Roose that sends Jon the letter. And the bluff could just be a way of trying to gather allies through fear, before anyone can learn of how weakened house Bolton has become.