Oh gosh wow. It might have worked like that in the past – especially during territorial colonisation / imperialist times, or when industrial production depended on the size of your manpower labour force – but it doesn't work like that anymore.
Things like sovereign funds, national assets, advanced technology and soft power determine a country's relative wealth and political power. Many of the top 10 most populous countries in the world are 3rd world countries.
I know it's an arbitrary ranking, but take a look at the top 10 passports in the world (as a measure of how these countries are politically regarded): Singapore, Japan, South Korea, Finland, Luxembourg, Denmark, Norway, etc usually feature at the top. All extremely small population wise – most have less than the population of London alone.
It's true that some countries become powerful by default, because of economic and historical factors in combination with a large population. However, the UK is never ever going to churn out enough babies to rival America, India or China, so I doubt that's part of the politicians' great imperialist masterplan...