OK, I measured the grip circumference (GC) and the grip length (GL) on the Citori Lightning camo and the Citori Hunter models, both with 3.5" chambers and 30" barrels:
GC was 4 15/16" on the Lightning vs 5" on the Hunter model.
GL was 4 3/8" on the Lightning vs 4" on the Hunter model,
Thus, the grip circumference is similar, but the grip length of the Lightning is somewhat (3/8") longer, which is probably better for shooting with heavy gloves.
I will post photos below as to how GC and GL are measured on a shotgun, and some side by side photos of the Lightning vs Hunter model, for eyeball grip comparison.
Finally, here is what Browning says about the Lightning grip:
Browning Citori Lightning Over and Under Shotgun -The lightning-style rounded pistol grip It offers a more open grip than full pistol grip designs. Many shooters feel this gives a slight speed-to-shoulder advantage. It represents a midpoint in design between ultra-speedy straight grips and extremely stable full pistol grips. If offers excellent control for whatever you are after -- fast-flying waterfowl, upland game or just shooting fun. Matching the rounded pistol grip is the forearm. It is nicely rounded at the fore-end and together, they give the name to this over under gun as "Lightning Style," or in this case, the Citori Lightning model.