Bonnets in the mid 1860s were small... very small. So small in fact, that often when I find extant bonnets, they are listed as children's bonnets. The bonnets in from 1865 to 1867 let the hair come out from under the back and do not come much further forward than the top of the head. On extant bonnets, there is a crescent shaped piece that replaces the bavolet that helps keep the bonnet up on the head. So don't be fooled when you see them, they really are for grown ups.
Godey’s Lady’s Book and Magazine
Description of Steel Fashion-Plate for March.
Fig. 1.—Dress of pearl gray silk reps, trimmed with green velvet arranged in the sheaf style. Paletôt of heavy brown silk, trimmed with wide and narrow velvet. Rose Portugal silk bonnet, made in the Empire style. Bands of pink velvet edged with white lace are attached to the bonnet and strap the waterfall. Gloves of buff undressed kid stitched with black.
Fig. 2.—Dress of brown poplinette, trimmed with black velvet and steel beads. The dress is looped up by black velvet rings over a petticoat of the same material as the dress. The corsage is made with a very long basque, and worn with a belt and sash richly trimmed with velvet. The hat is of black straw, trimmed with black feathers, and a long black veil of spotted tulle which is turned round the hat and falls over the back.
Fig. 3.—Dress of pearl-colored gros grains, gored and trimmed with Magenta velvet and buttons. The bonnet is of pearl Terry velvet, trimmed with a long plume, crystal ornaments, and Magenta velvet. A long veil of white tulle is fastened on the right side of the bonnet.
Fig. 4.—Full suit of mauve linsey, trimmed with heavy silk cord and white llama fringe. The bonnet is of white silk laid in folds, and trimmed with mauve velvet. A long veil of white tulle covers the face.
Fig. 5.—Dress of white alpaca, trimmed with bands of blue silk edged with black lace. A wide sash of bias silk falls over the back of the dress. Black silk paletôt, made very short in front, and richly trimmed with bugle lace, cord, and buttons. Black Neapolitan bonnet, trimmed with blue velvet and white field daisies.