The only reason it's not realised just how strong women were in the past is because a) nobody other than the odd (in all senses of the word) middle or upper class man was particularly interested in recording what women did or looked like if it wasn't on their backs and b) clothing covered the body, including the upper arms so nobody really saw muscle development when photography became the new hobby.
Think about it - a galvanised 5 gallon bucket weighs about 20kg. A milk churn would often be 11 gallons, so around 54kg. Then the work of scrubbing dairy floors, moving cows, looking after livestock, shifting whole cheeses, etc, etc. A milkmaid wasn't just fetishised because she was clean, well nourished, unlikely to get smallpox or was very skilled in particular hand movements - she would also have been muscular, strong and healthy.
Pit brow women were absolutely stacked (they were banned from working underground in Victorian times because the idea of not wearing a lot of clothes and trousers in particular, really upset the upper classes). They were still employed until the early 1970s.
Women would climb and abseil cliffs with just a rope and baskets to collect gulls' eggs or reach shellfish - the strength needed for that is phenomenal.
Even those who didn't do an exceptionally physical job would be carrying coal scuttles, fetching water from wells and pumps, sweeping, mopping, scrubbing, handwashing, beating carpets, preparing food, carrying shopping, lifting children, caring for animals, walking miles - if they were fortunate enough to have access to reasonable nutrition, they would have built muscle on backs, arms, shoulders, hips, thighs, calves, everywhere. And don't forget riding - it takes real core and leg strength to be able to ride, whether sidesaddle or astride (which was normal, there is thousands of years of clothing designed for women that showed they rode normally before the invention of the double pommel saddle.
It's very possible that the narrow waists on clothes from earlier times wasn't solely because women were 'thin' and wore corsets - it was because they had very strong core muscles/could even have been to emulate the body proportions of those women who worked. As, after all, the clothes that have been preserved will be the ones of wealthy women, not working ones.
The Reubenesque 'soft' women were different because they showed wealth (as in more than adequate food and no exposure to sunlight). But if you actually look at the paintings, such as in The Judgement of Paris, there are depictions of defined musculature in their backs, arms, shoulders, thighs, calves and backsides. And, of course, they were of actual women, rather than of male models with breasts painted on. If you look at Raphael's study for the Alba Madonna, the woman has defined shoulders and amazing thigh musculature.
In short, if you actually look at art beyond the portraits of fancy dresses and think about what life would have actually been like, women have always been both feminine and strong/muscled in a way the less well-informed are unaware of.