Expenses are usually divided up equally between all adult tenants with access to student loans, regular source of employment income or savings.
Sometimes individual tenants share of rent or bills is discounted to reflect special circumstances or alternative household arrangements.
For example: dog owner agrees to pay more rent to cover occupying the largest bedroom with the dog, pay for all damage caused by their pet and may be willing to pay a larger share of the bills than everyone else to encourage other tenants to look after/feed/walk their dog.