You should never start a startup in a massive or over saturated market. You want to create a unique product and start with a small niche before scaling. Large and existing markets usually mean that you have tons of competition. It will be very difficult if not impossible to differentiate your product from other similar products/services.
Your focus should be on creating that unique product or service. You want to be a one of a kind company where it is the only one in the small ecosystem. You do not want to be the 10,000th makeup artist or the 10th photographer on your street.
Go after small markets that people don't even think exist, then scale and turn it into a monopoly business. You can get a foothold quickly and become a force to be reckoned with in the future.
As a startup founder, you shouldn't willingly go out looking for risks to take on; such as getting into a saturated market. Take competition as a form of validation; when we go for things that a lot of other people are going for, is a futile risk in itself. When lots of people are doing one thing, it is often proof of insanity and it would be difficult to stand out.
Going for a competitive market makes the startup growth stunted. The company may scale well at the beginning but growth soon stops as industry giants monopolise the business. Starting first has its advantages but you must always have the future in mind.