Contributing Area
The size of the contributing area of the rainfall in a basin has a direct influence on the total volume of runoff that drains from that basin.
It probably comes as no surprise that when rain falls in a uniform manner over a larger basin and a smaller basin, the larger basin produces more runoff volume. All things being equal, a drainage area that is twice as large can generate twice as much runoff volume as its smaller counterpart.
Of course many storms will cover only part of a basin. So for most situations, the runoff volume will be determined by the contributing area – that part of the basin covered by a storm – not the total size of a basin.