Financially that isn't always the case. If you want to make more money, you need to work in an environment that has a larger volume. An established group can offer a more attractive salary because there is already a sustained volume of patients seen from the last 3 years. After a patient hasn't been seen in three years, the patient is considered a new patient.
A solo practice can be lucrative but it will take several years to build up the volume to make it financially rewarding.
For most specialties the best (for specialized care) is a physician group of 5-13. That way you can have projected monthly revenue of around $400k to $1 million a month. Depending on how the group manages the money, the clinic could pay for your insurance. If you run a solo practice, you will need to flip the bill all on your own.