I have been to rural country in China. The animals lived in stalls very close to the farmers house. They fed animals anything they have, left over, grass stalk of grain etc.
That's why the most prevalent domestic animal (not poultry) was pig. Pig eats everything, including human refuse . . . the same reason why Judeo-Christian-Islamic cultures prohibits pork.
It's not entirely impossible for Rome to have 50 million people, but that's just too much population for Rome's own good. A preindustrial society can have 1 billion people, like China and India, but it means the vast majority of population have to live in desperate poverty. By the time of late republic, the population was probably 10 to 20 million, Rome was already periled with famine and revolution. A healthy population for Rome was probably 1 - 2million, within Italy and 5 million for the whole empire. Similarly, for preindustrial China, the best population was 100 to 200 million like that from Song to Ming. It was a healthy and vibrant society. But when the population was stretched to above 400 million like late Qing, Chinese society became lifeless, stagnant and a hot bed for famines, civil wars and revolutions.
Ming was just as lifeless, stagnant and full of internal unrest . . . in fact, it was overthrown by a peasant rebellion due to the government incompetence in managing taxation levels according to changes in economic and monetary reality. The fundamental problem was/is not the total number of people, but what the society did/does to handle the "excess" population when the most efficient famers could/can produce more than they could/can consume themselves. The "excess" is not really the Malthusian concept of going beyond the land's carry-capacity . . . but almost the exact opposite: as farm productivity increase, there is no need to devote 100% of the population to subsistence farming; what to do with the population who can not compete in the market place when the agro product price drops? Roman approached the problem with direct government handouts, when the arrival of grain from Egypt must have driven many of the local farms in Italy out of business, in addition to the hands who were not needed on the local farms to begin with (even Italian farms were beginning to have surpluses before the arrival of Egyption import). Government handouts of course meant than an even greater quantity of goods than that which was handed out had to be confiscated from producers to begin with. China apoached the problem by pushing population onto poorer and poorer lands, and periodicly disrupt farm productivity growth through the break-up of successful farms that were buying out their less successful neighbors. Both policies led to hopeless stagnations as vast numbers of human labor were essentially laid to waste either not working at all or working inefficiently subsidized by government transfers at the expense of more efficient farmers. Efficient farmers were discouraged from improving as the fruit of their labor was confiscated by the government through taxation, inflation and outright "equalization" attemps. Sooner or later some kind of disaster struck, and the society simply did not have the reserve capacity to handle it.
Eventually, a splintered Europe in the 1700-1800's figured out a way to get out of this lower level equilibrium, and figured out how to utilize hands that were no longered needed on the farms: commerce and manufacturing. So that the "excess" labor in the agro sector could do something else to pay for the food that they needed. The splintered Euopean governments simply did not have the power to either run a bread-and-circus like Romans did, or to engage in "levelling" or bureacratic transfers on a vast scale like Chiense did. The people who got "laid off" from the farms due to producitivity increases had to look for real productive jobs to earn their daily bread! That made new experiments in commerce and manufacturing possible as they all required labor, and initially relatively cheap ones because the business methods were immature at the beginning. Subsequent commercial gains lured the intellectuals away from scholarsticism based on scriptures (which was temporizing but fundamentally non-productive just like the old Chinese exam system based on scriptures); that led to modern science and technology, which eventually made farms far more productive, which in a couple more centuries made the vast majority of the population available for endeavors other than farming. Interestingly enough, China stumbled upon the value of commerce in Soutern Song, when the country was splintered, and the high farm output of the south did not have to be confiscated by bureacratic transfers to the north. The brief period of baby steps towards industrial take-off in China was cut short when the bureacrats and government sponsored opinion makers in the government universities insisted on fighting the Jurchens and only to have a far more dangerous band of nomads the Mongols coming up close to the door steps.