Big companies are not your worry in most cases. They have so much bureaucracy and internal politics that it's often cheaper for them to buy out the up-and-coming startups than it is to build in-house.

Instead, if you achieve success without much of a moat, you will be cloned by one of the "App Store mills". These are companies who employ multiple small teams (think 3-4 people), and each team is charged with cloning one of the most popular apps - they have really abbreviated development cycles, on the order of 6 weeks, and they just keep churning till they've drowned every app on the leaderboard in clones. And because they are astroturfing the whole space, they don't actually need most of these clones to be individually profitable.

Rocket Internet Is one of these copycat business style.