In simple terms, Petra, if you have an increased population and low unemployment (so an increase in people earning) then there will be an increase in demand for services and consumer goods which means higher sales and increased profits.
Even an increase in public sector spending to provide health, education, police services etc to the increased population will have the same effect. The money spent on public services doesn't just disappear into a black hole, it goes on infrastructure, resources etc. All of which are provided by private enterprise and are a source of profit (and tax). Not to mention the spending of their wages by the public sector workers.
Of course, the fly in the ointment is the fact that the biggest companies that profit from all this increased spending like to squirrel away as much of their profits as they can into offshore tax havens, so money is leaching out of the country instead of staying in circulation for everyone's benefit..
And, of course, that we don't have much of a manufacturing base any more so we tend to but imported goods..