In "A Thousand Acres", why do feminists claim Smiley's novel as a statement about the diminution of women?

In this novel, the central focus is on the sisters, Ginny and Rose, with an emphasis on Ginny and all of the unsettling realizations that she discovers about the calm, steady life that she thought that she was living.

Ginny spent most of her life as a housewife, going about her housewife and farmer's wife duties, living quietly at the behest of her husband and rather overbearing and stern father.  As the novel progresses, her remembrance of her father's abuse of her growing up, her father's more stark emotional and mental manipulation of them in regards to the farm, her affair with Jess, and her eventual complete departure from the life that she knew all point to the fact that she lived a repressed, stifled life that kept her from reaching her true happiness and potential.  She had spent her entire life not questioning anything, and letting the males around her decide how life was going to be.  That minimizes the role of a woman in all aspects of life.

Another woman to look at is Rose, who is more proactive about her happiness before Ginny even takes her own steps in that direction.  She struggles with her marriage, is fiercely bitter against her father's cruelty and overly-partriarchal behaviors, and to escape all of it, she leaves her husband to be with Jess, farms in a way that she knows will completely upset her father, and generally stands up to all of the repression that other males had wanted to put upon her.  She too comes out from under the shell of being minimized in importance and takes life into her own hands, fashioning it to be how she really wants it to be.

Then, there is Caroline, who is completely ostracized and cut out of the inheritance, simply because from the very beginning, she wanted to live a different lifestyle than the one her father imagined for her.  Caroline's worth is completely devalued by her father, because she doesn't fit into his very rigid outline of what a good daughter and woman should do.

"A Thousand Acres" is a very difficult tale of women who, in order to feel importance, value and validation in their lives, have to completely reshape their existence, often at the behest of those around them. I hope that those thoughts helped; good luck!