This is because the positive side of a large population is that they can provideĀ labour for theĀ entire country and without labour which is a main factor of production nothing could be produced.
Yes population is said to be the driving force of the nation because lots of population means lots of labour force Plus defence and lots of other sectors. a historical event connected to this question maybe the industrial revolution in Europe in the mediaeval ages who wanted a large population to become more strong then other countries thus population can be said as the main factor to be driving the force of an Nation...