In which country/s, in which period/s? Because "before the 20th century" covers everything from Pharaonic Egypt and Imperial China to the Ottoman Empire and the Mayans. You're treading awfully close to an "in your era" or "throughout history" question.

In the past, obesity wasn't looked at in the same way that it is today. Since food was generally scarce for the people, obesity was seen as a sign of wealth and prosperity. If you were rich you could afford lots of food and slaves or servants to do your wishes so you wouldn't have to lift a finger. The poorer people would have a limited amount of food and would have to exercise a lot more, like walking everywhere and working in heavy labour jobs. It was a sign of social status, and for women it was a sign of health which would bring forth many children. It's the reason why kings and the like were obese. It's also why they didn't live very long or healthy lives.

It was actually in Ancient Egypt that obesity started to be known as a disease. The Greeks and Romans also believed it to be an illness, saying it caused infertility and death. The Ancient Greek physician Hippocrates said, "sudden death is more common in those who are naturally fat than in the lean." They realized that eating too much was a bad idea, however they didn't realize that exercise and diet were needed to balance their weight until the 1600s. Instead they "vomited and purged themselves three times a month." Kind of gross, but effective in their minds.

I hope this helps!