I agree it demonstrates he is good at being a big fish. Both Andy Burnham (Greater Manchester) and Ben Houchen (Tees Valley) have often been given "Big Fish" status. Houchen known for using mayoral development corporations to take control of major industrial regeneration projects in Tees Valley. Burnham widely seen as a figure who has used the role to become a major political player, sometimes acting as a counter weight to Westminster.
I accept you personally view the mayoral regions - London, Greater Manchester, Liverpool City Region, West Midlands, West Yorkshire, South Yorkshire, Tees Valley, North East, West of England et al- as "small ponds".
You seem to infer there it maybe somehow lesser in having 10 successful years, winning 3 elections, by being a "big fish" Regional Mayor in a "small pond" region compared to other central Westminster politicians- and of course you are entitled to such an opinion.