Is worsening wealth inequality intrinsic to Capitalism?
While the owners of a successful business will generally experience an exponential growth compared to their employees, there are some factors to consider:


I find it unrealistic for any business to maintain a growth rate of 100% over many years. (As per the example) You can only sell twice as much if you can find twice as many buyers.
As a business grows, it is likely to start hiring more people and paying them better as they need better skills.
The business also takes the risk. The worker is generally guaranteed his X, but stockholders may have to expect the occasional decline in profit, or even a loss. The company may even go out of business in which case they cannot recover much, if any, of their investment. Those who take the risks, get the reward. Businesses failing also mean that bad investors can often lose fortunes, it's not always a good investment.
There is nothing stopping the employee from owning part of the company by investing in some shares. Then he too may experience this exponential growth. He must also keep in mind, however, that he must now also accept the risks of such an investment.