This is a definitive answer as related to me right from an insider at Browning years ago. A vernier micrometer and an accurate weight scale will bear it out. I own both gauges (16 & 20) and have actually measured them side by side as well as weighed them. The 16 Ga Citori is built on a modified 20 ga frame that has been widened by .100" to allow for the wider barrel block necessitated by the slightly bigger chambers. The height is exactly the same as a 20 ga., because the 16 ga chambers fit well within the allowable space vertically.
Weightwise, the standard 16 ga Citori Lightning is marginally lighter than the 20 ga guns overall. The biggest factor is the lighter barrel block area due to the bigger chambers. (The 20 gauge is lighter than the 28 and .420 models for the same reason). There is less wood in the forend and the butt stocks are about identical except for a bit more wood width wise in the cheeks to match the wider receiver block.
