What happens that if both a and n are large than the new a obtained after multiplying with suitable power of 2 becomes very large and leads to overflow.
Notice that at last what u are doing is divion.The final answer is a/b or b/a whichever of a or b is greater.so multiplying b by 2(n/2) and a by 2(n/2+ 1) is same as multiplying a by 2 only once and then final answer is max(a,b)/min(a,b).