The reason it’s a logical fallacy to assume size makes a real difference is the work scales with population, but the capacity to solve problems also scales with population. That’s a a formal fallacy in that the argument doesn’t follow from the premise.
In general economies of scale make things easier as you start dealing with ever larger groups.
> the capacity to solve problems also scales with population
Missing a potential key factor is not a logical fallacy. You can be perfectly logical operating from the wrong set of underlying assumptions and be completely wrong.
I'd say that your statement sounds intuitive but is not necessarily correct. As a cell's size increases, its surface area and volume both increase, but its surface area increases by the square of the radius while the volume increases by the cube.
The problem set and the solving power both increase as population size grows, but I would say problem set almost certainly increases faster. That's why people break populations down into smaller groups to be managed (federal, state, county, city, HOA).
As technology improves we may be improving the rate at which our solution power scales with population, but I think it's likely we have a long way to go until it scales faster.
It’s not missing a factor that was an issue, but jumping from size to difficulty without justification.
“All cats are mammals therefore all cats are blue.” The conclusion being false isn’t a logical fallacy. However because the conclusion isn’t supported by the argument it is a fallacy.
It common to see this kind of fallacy for example: “Smart people learn things faster, therefore all experts are smart.” Yes, you just said two things that seem related, but that doesn’t mean the first statement implies the second.
In general economies of scale make things easier as you start dealing with ever larger groups.