Living "modestly" is important to the Amish and women's attire is a big part of their commitment to that code. (One could say that it is male-imposed, because the Amish are profoundly patriarchal in their dicta. In their 'councils' though, the women do have a voice.)

The Amish devolved from Roman Catholics (whom they now fear and despise) and their garments reflect that influence. It's interesting to me that they modeled their women's clothing on traditional nuns' attire.

It varies a great deal from church to church but this includes longish dresses, compressed breasts, doubled material in the chest area, long sleeves, thickish dark stockings, the symbolic head covering that in turn is encased in an outer 'bonnet' that casts the face into shadow. The whole intent is to avoid flamboyance in any sense.

When my family moved from Oregon and joined the Amish church in Virginia, my mother continued making her daughters' dresses considerably shorter than the preferred standard in the 'new' church. Years later my mother wrote an apology to the Virginia church acknowledging that she should have adhered to the standards of the church that she joined.