We present spectroscopic and optical studies of a non-layer-shrinkage ferroelectric liquid crystal DSiKN65. The orientational order parameters S, measured with respect to the smectic layer normal using IR spectroscopy on a sample aligned homeotropically, does not exhibit any significant variation between the smectic-A∗ and smectic-C∗ phases. In contrast the birefringence of a planar homogenous sample abruptly increases at the smectic-A∗ to smectic-C∗ transition. This suggests a general increase in the orientational order, which can be described by the orientational order parameters S' defined with respect to the director. Simultaneous increase of S' and the director tilt Θ may explain the low shrinkage of smectic layers, which is consistent with recent theoretical models describing the smectic-A∗ to smectic-C∗ transition for such materials.