...Basically people who were wealthier, ate kind of fattier meals consisting of sugar, carbs and meats (because sugar and spices were a more expensive commodity at the time, and it was kind of a status thing). If you were plump in those days, it presumably just meant you were well-fed and well off. But for peasants, cows and chickens were household treasures for their milk and eggs and they wouldn't dare eat them except on special occasions (eg. a hunting festival), so most of their diet was stuff that were plentiful and forageable.
Can you imagine how the times have reversed now where chicken is a cheap meal, but seafood like crab and lobster are twice as expensive?
I think it's kind of refreshing they didn't make her lose weight automatically and be skinny in like 5 chapters, and it seems she's genuinely working out very hard each day.
Anyway, I'm not really sure how big she would be in reality, but having a bit of body weight in Historic European culture was actually not uncommon because their diet consisted of really high-calorie foods (in actuality, peasants ate much healthier meals consisting of fish, seafood, and other shrubberies). But given this is a Korean manwha, maybe the author didn't research very much. In most historic paintings, the women depicted are actually quite plump, but I'm sure ridicule for being overweight has also existed for centuries.