A large chunk of that is simply differences in population sizes. The current US population is likely larger than the global population in 1,000 CE. Further, children used to make up a larger chunk of the population exaggerating the differences.
So, it was often possible to scale up production even more, but there where not enough customers to justify it.
The vast majority of limited trade during pre-industrial times is due to the inefficiency of (i.e. cost) of long range transportation. A single modern cargo ship can carry more cargo in a single voyage than an entire years worth of the entire medieval world's cargo fleets. And at a price that is few orders of magnitude cheaper (per ton).
Not that it mattered since production of everything was, by today's standards, a few orders of magnitude less efficient.
It's often difficult to fully grasp how inefficient and slow everything was in pre-industrial times until you dig into the details.
So, it was often possible to scale up production even more, but there where not enough customers to justify it.