In the late 1800s, the architecture of the shoulder in women's apparel reached massive proportions, achieved by using voluminous fabric with tucks and pleats for support....heavier fabrics often had sculpting pads at the seam. The Victorian leg-o-mutton sleeve is a prime example.
Over the next 30 years, shoulders were left natural, often given attention by being revealed with open designs but not exaggerated as before. By the 30s women's apparel had become long, lean and feminine with more attention to sleeves, the back and neckline than to shoulders. But by the late 30s the shoulder started to assert itself again with demure ruffled caps, slight angles and dainty puffed sleeves.
Maybe its coincidence, but at this point as women's fashion was starting to embrace the masculine and empower them a little with a broad shouldered look, globally we were seeing the threat of WWII building. By the time women in America saw their husbands off to war and stepped up to the workplace en masse, the shoulder had reached unusually large proportions. Women dominated the workplace as well as the silver screen and wore the shoulders to prove it.
After WWII Christian Dior reigned in those shoulders and with the lifting of restrictions on fabric, put the emphasis back on the female form in a more traditional sense, exaggerating the bust and hips with nipped waists and massive full skirts. The look became the silhouette of the 50s to mid 60s, when another fashion revolution mimicking the 20s removed the restrictions of the wasp-waist and gave women the freedom to dress without a girdle. By the end of the 70s, the 30s influence became strong again...long, lean & feminine. In the 70s too, women were steadily gaining in the workplace and achieving things unseen before, finding their place in the white collar world long dominated by men. Coincidentally or not, the fashion world once again began to empower women by giving them the big broad shoulders the men they were competing with came by naturally.
I'm sure in retrospect there will be a reason (other than the predictable cyclical nature of fashion itself) that the large or sculpted shoulder is making a comeback at this time in history. Wars, a new president, the recession....certainly no lack of reasons for women to feel like they need a little help shouldering the woes of the world. Regardless, they're back. Whether you're ready to go large or not, the attention for a while is going to be on the shoulders!