A monopoly business gets stronger as it gets bigger: the fixed costs of creating a product(engineering,management,office space) can be spread out over ever greater quantities of sales. Software startups can enjoy especially dramatic economies of scale because the marginal cost of producing another copy of the product is close to zero.
Many businesses gain only limited advantages as they grow to large scale. Service businesses expecially are difficult to make monopolies. If you own a yoga studio, for example, youll only be able to serve a certain number of customers. You can hire more instructors and expand to more locations, but your margins will remain fairly low and youll never reach a point where a core group of talented people can provide something of value to millions of separate clients, as software engineers are able to do.
A good startup should have the potential for great scale built into its first design. Twitter already has more than 250 million users today. It doesnt need to add too many customized features in order to acquire more, and theres no inherent reason why it should ever stop growing.