IMHO, both situations are true. There used to be more regulations in the US to prevent monopoly formations in media in a region [1] that once relaxed lead to more concentration. Banks had/have some regulations on M&A [2] which when relaxed generally result in smaller banks getting bought out. Concentration are natural trends in many markets without regulation to limit it. Maintaining the "invisible hand of free markets" isn't actually a naturally stable equilibrium in many markets. Especially those where fungibility is low (local media, local banking, ISPs, healthcare, etc).
Now in other markets a high bar of regulations conversely encourage concentration due to increase costs of meeting the regulations. Reforms like Frank-Dodds can be a mix, both making it more expensive to meet the accounting and reporting needs favoring larger companies, but also imposing rules limiting concentration of ownership. However regulation heavy fields can still be opened up by startups/new entrants if they're significantly better than competitors (SpaceX comes to mind).
It's multi-faceted game theory, not a simple rule or sliding scale.
Now in other markets a high bar of regulations conversely encourage concentration due to increase costs of meeting the regulations. Reforms like Frank-Dodds can be a mix, both making it more expensive to meet the accounting and reporting needs favoring larger companies, but also imposing rules limiting concentration of ownership. However regulation heavy fields can still be opened up by startups/new entrants if they're significantly better than competitors (SpaceX comes to mind).
It's multi-faceted game theory, not a simple rule or sliding scale.
1: https://www.mofo.com/resources/insights/210503-fcc-relaxed-m... 2: https://www.fdic.gov/regulations/laws/rules/5000-1200.html