When it landed on the ground in the Air temple place, maybe it was heavy enough to crack the floor beneath it.

The floor may have been cracking beneath its weight all this while, and as the cracks got larger and larger the floor under the statue (or, from our heroes' point of view, the ceiling above their heads) got weaker and weaker until the statue finally broke through and fell.

At least, I assume that's what happened. Either way, I don't really think the Giant needs to calculate every single thing that happens in his strip. Like he says, things move at the speed of plot.