Super-simplified explanation: Three things matter for carrying capacity.
1) How many calories does a human need to survive?
2) How many calories can we squeeze out of an acre of arable land?
3) How many acres of arable land are there?
The first one is pretty much immutable.
The third one is maybe somewhat mutable, but to a very limited extent. We can squabble how much, but let's assume it's fairly immutable. Large changes would need energy amounts that are currently prohibitive.
That leaves calories per acre. And that's where ag-industrial methods had an impact that's absolutely bonkers[1].
As the graph shows, this is not a fixed number - that's why Malthus' predictions ultimately didn't come to pass - but you need ag-industry to squeeze out those numbers. (Without mechanized agriculture, massive production of artificial fertilizer, and Borlaug's dwarf wheat, Earth would have run into massive hunger by the end of the 80s - look for "Green Revolution" for lots of info on that)
And small scale subsistence farming (which is what most folks think of when the say "I want to grow food") cannot utilize all of those techniques.
That's also where the "4 billion" number comes from - we know how many calories/acre non-industrial farming generated, and it was at best enough calories for ~4 billion people. Optimistically.
1) How many calories does a human need to survive?
2) How many calories can we squeeze out of an acre of arable land?
3) How many acres of arable land are there?
The first one is pretty much immutable.
The third one is maybe somewhat mutable, but to a very limited extent. We can squabble how much, but let's assume it's fairly immutable. Large changes would need energy amounts that are currently prohibitive.
That leaves calories per acre. And that's where ag-industrial methods had an impact that's absolutely bonkers[1].
As the graph shows, this is not a fixed number - that's why Malthus' predictions ultimately didn't come to pass - but you need ag-industry to squeeze out those numbers. (Without mechanized agriculture, massive production of artificial fertilizer, and Borlaug's dwarf wheat, Earth would have run into massive hunger by the end of the 80s - look for "Green Revolution" for lots of info on that)
And small scale subsistence farming (which is what most folks think of when the say "I want to grow food") cannot utilize all of those techniques.
That's also where the "4 billion" number comes from - we know how many calories/acre non-industrial farming generated, and it was at best enough calories for ~4 billion people. Optimistically.
[1] https://www.researchgate.net/figure/USDA-per-acre-crop-produ...