It grows until it consumes so many nutrients that the other living cells don't get enough. The host literally starves even if he eats plentifully.
The same applies for the US: The billionaires are not only hoarding wealth, but by doing so they're crippling the economy for workers and everybody besides themselves.
Most people aren't frivilously spending most of their money on things... Except maybe the rich, but most people don't have that much disposable income now do they?