Iron ore prices have slumped to near-record lows but the four biggest producers of the metal, Rio Tinto, BHP Billiton, Vale and Fortescue Metals Group have continued with the same rate of production through the slump.

The four companies produced 71 percent of the world’s supply of iron ore in 2014 and that number could rise to 80 percent as smaller suppliers struggle to survive the low prices and the growing supply glut.

The Wall Street Journal reported that BHP said its iron-ore output rose 14 percent to a record 233 Mt (256 million st) in its fiscal year through June, and forecast another 6 percent rise in the year ahead. Rio Tinto said it is sticking by plans to increase its output from the key Pilbara mining region of Western Australia to 360 Mt (396 million st) within the next couple of years, up from around 280 Mt (308 million st) in 2014. Vale meanwhile aims to produce 450 Mt (496 million st) of iron ore by 2018, up by more than a third from its output in 2014.

Before the early 2000s, low prices and big investment costs meant the iron-ore industry was dominated by a small handful of large producers. From 1980 to 2005, iron-ore exports from Australia fetched an average $30 a ton in today’s dollars, BHP says.

Thereafter, China’s breakneck growth sent demand and prices skyrocketing, luring in new rival suppliers. The top producers’ market dominance eased, but prices surged to more than $190 a ton at their 2011 peak.

Now, as China’s demand growth has waned, prices have fallen back to $52 a ton, close to their lowest level for a decade. The majors’ gamble, once again, is that it is better to produce as much ore as possible even at a low price, rather than pump out less but potentially enjoy a higher selling price as the market tightens.

The majors’ operating costs are low enough for them to keep a reasonable margin on each ton they ship even with lackluster prices, thanks to their economies of scale. The majors also don’t want to cede market share to their big rivals by cutting production. Anti-cartel rules meanwhile prevent from them from using their oligopolistic position to collude to restrict supply.

But investors and analysts have become more critical. BHP reported a 35 percent year-over-year fall in profit from its iron-ore operations in the six months ended in December. It will report full-year results next month.

Both BHP and Rio are generating their lowest level of returns on equity since the 1980s, Citi recently estimated: each company’s shares are trading close to six-year lows.

The debate reached a flash point in May, when smaller rivals, squeezed by plummeting prices, called on the Australian government to hold a parliamentary inquiry into the industry’s relentless output expansion. The government rejected the calls.

Critics of BHP and Rio Tinto included the U.S. miner Cliffs Natural Resources Inc. and Fortescue, which has itself hiked production sharply in recent years, but is grappling with a massive debt pile and higher costs than its Australian rivals.