Hoping to tame soaring healthcare costs and prevent unfair competition, Massachusetts regulators have significantly strengthened their oversight of medical building projects. The measures could substantially constrain Boston teaching hospitals from colonizing the suburbs, territory viewed by the city hospitals as prime terrain for growth. Companies that want to open outpatient clinics costing more than $25 million will have to prove to state officials that their services won't duplicate what's already there and won't imperil existing facilities. All physician-owned day surgery centers will undergo the same review.