I'm creating a post-apocalyptic world several centuries into the future. After the nuclear war whatever is left of humanity has regressed into the dark age.
The offspring of those few who survived the radiation and the following nuclear winter live in small tribes. They sometimes cooperate, and sometimes compete with other tribes. Humans are few in number.
Technology regressed to the dark ages. The survivors hate technology and blame it for the fall. Anyone caught trying to learn or recover the before-the-fall knowledge is ostracized or put to death. Whatever is left from the destroyed civilization has been overtaken by mother nature like in Life After People. Rotting machines are mostly used for scrap metals by village blacksmiths.
Food production is limited to small scale horticulture and herding. Hunting and fishing are very important food sources since there are many animals and very few humans.
Many places are highly radioactive and must be avoided. Some are mildly radioactive and people pass through them quickly if they are forced to. There are many dangerous mutated animals. Humans who show signs of mutations are killed.
The climate is much colder, with earth just recovering from an ice age. Humanity must rely on many food sources, since climate is very unpredictable.
In my story the average man is twice as large as average women. While the differences in modern world are 10-15kg, the average 25 year old woman in my setting weighs around 65kg, while the average 25 year old man is around 130kg. Obesity is unheard of. Men are bulky; not like modern body builders but like people who gained their muscles from a lifetime of hard work.
Is there a way to explain why dimorphism increased dramatically without resorting to genetic engineering or radioactive mutations?