This is the economic argument idiots routinely make about immigration. More immigrants means more jobs doing stuff for immigrants! What it leaves out is that immigrants need jobs or else they're on welfare. So while bringing in hundreds of thousands of people increases the size of the demand, and thus jobs, it also increases the number of people needing jobs.
To put it language even a simpleton can understand. If you have dinner and invite your neighbour's family to dinner and he brings food, you have more food! But you have more people consuming it. So it's not like there's actually more food for you. The table is just more crowded now. Not to mention the bathroom.
This is the arguments racist use