For anyone who's wondering where this question came from, and why sixteen tonnes specifically: https://en.m.wikipedia.org/wiki/Sixteen_Tons

The song lyrics actually say load 16 tons, not mine 16 tons. I don't think a single human could mine 32,000 pounds of coal in a day. But a whole crew could possibly? Not sure. If this guy's job was to operate some sort of loader machinery that would make more sense.

The thing that's mentioned in the song is "sold my soul to the company store" which identifies a big source of the problem. Coal miners were often paid in "company scrip" basically a form of currency that's only redeemable at a store the company owns, which often had massive markups. In addition, often there would be "company towns" where all the workers would have to live, and pay rent to the company, which was frequently well above market rent.

One famous example (not coal, but industrial) is Pullman, named after its founder, George Pullman who employed a lot of these tactics, and was so universally hated that when he died, his family had his casket entombed in concrete and rebar because they were so worried the former workers would desecrate his grave.

According to industry publication Coal Age:

On November 12, 1917, the Orient mine made its record run (which is also the record for this field) of 5,508 tons of coal dumped in eight hours—this on a single-cage hoist. On the day this writer was at the plant, 4,900 tons were hoisted in eight hours; this coal was loaded by 441 miners, or an average of about 11 tons per loader,” world-class numbers at the time.

This was around the time Travis was born, so advents in mechanical mining may have advanced by the time he landed on 16 tons specifically, though it would seem as if that would still qualify as a very large amount of coal for a single person to load.

I think the perpetual debt is the issue at the heart of the song, rather than the specific amount of coal that one would be held to. Merle Travis's original version of the song has an informative preamble.

Yessir, there's a-many a Kentucky coal miner that pretty near owes his soul to the company store. He gets so far in debt to the coal company he's a-workin' fer that he goes on fer years without being paid one red cent in real honest-to-goodness money. But he can always go to the company store and draw flickers or scrip -- you know, that's little brass coins that you can't spend nowhere only at the company store. So they add that against his account and every day he gets a little farther in debt. That sounds pretty bad, but even that's got a brighter side to it.

Travis was born into a coal-rich region of Kentucky and many of his songs deal with the realities of that industry. See also "Dark as a Dungeon".

You're working under the assumption that the miners are well compensated, or see some sort of wage differential based on how much they mined.

Miners historically have been underpaid. It is dirty, dangerous work, and those doing the work weren't paid well. Through out history, slaves typically were the ones working the mines. In more recent history, the poorest people worked the mines because they were often one of the only employers in the area. Many mining companies also would own nearly everything in town, from the houses the workers stayed in, to the store they bought their supplies at, and the bars they drank at. Everything was marked up, so the miner couldn't afford to do much, because if a person could save up enough they would use it to seek better employment.