轻巧龙身体长而纤细,颈部也很长。对于轻龙的了解主要来自于单一副保存完好、但缺乏头骨的颅后骨骼。与其身长比起来腿的比例较短,在兽脚类中稍显独特。葛瑞格利·保罗(英语:Gregory S. Paul)(1988)表示这是他鉴定过身体最长、胸腔最浅的兽脚类。[3]轻龙身长约6.2米,臀高1.46米,体重约210公斤。[3]2016的另一份估计值为身长7.5米、臀高2.1米、体重210公斤。[7]轻龙胫骨测量长60.8公分,长于股骨的52公分,跖骨的长度是股骨长度的74%。这样的比例与某些似鸟龙类相似,显示轻龙可能也善于奔跑。[8]长尾部末端下弯,是很罕见的情况,可能非埋藏学所造成。虽然轻龙的颈部很长,但关节突(英语:Articular processes)纤细且颈椎缺乏上骺显示其灵活度远不及其他兽脚类,并可能只能支撑一颗很小的头颅。此特征反驳了轻龙是大型掠食者的假设,因为与泥潭龙亲缘关系接近,牠很可能也是植食性或杂食性动物。[9]
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