As others have said, it can be family money, early inheritances, buying young etc. Not everyone with a big house has the best job, and vice versa.
My uni friend with the highest paying job (Mgmt consultant in SE, no DC) lives in a standard 4 bed house on outskirts of London, even though she could afford a mansion!
Another friends sister recently bought a second home abroad. I know that their mum gave them a lump sum when she downsized a few years ago, but the sister has also been given another lump sum towards the second home, with the mum’s will making things equal for siblings! To outsiders, the sister will seem minted, but much of the money has come from her mum.
We have a 4 bed house in NW, smaller than some, bigger than others. We both on above average but not stellar salaries, but the difference is we first bought - separately - when young (now mid fifties) so have been mortgage free for quite awhile.
Despite comments from others with bigger houses, we are not bothered about moving to a bigger house, although we have extended. We prefer that our DC will benefit when they buy, so we are passing money onto them already, plus they have an inheritance from a distant relative.