Where does the link between population growth and GDP per capita growth come from? If growth of GDP per capita is caused by growth of population then why is Ind

Where does the link between population growth and GDP per capita growth come from? If growth of GDP per capita is caused by growth of population then why is India so much poorer than Switzerland? Uganda and Afghanistan both have very high growth rates too, yet they're both very poor. I doubt that increasing immigration that much would actually cause any increase in GDP per capita at all - the more you increase immigration, the lower standards you have, and if you want to have 700 million more people by the end of the century you'll get 10 million a year, a lot of whom are unskilled non-English speakers since there aren't enough highly skilled potential immigrants to fit the demand. What, exactly, would they be doing that would be increasing the GDP by that much? I think it's much more likely that technological improvements are the main cause of increasing GDP per capita, and the increasing population is a side effect.