Here's something I've been thinking about for a while and I'm struggling to figure it out. How do rod weights (in terms of AFTMA grain weights) relate to fish size in terms of two rods from the same series but of different weights? For example, a 12wt weight "requires" 380 grains to load, and it is common and acceptable to fish for and land 100lb tarpon on such a rod, yet a two weight at 80 grains (approximately 20% of the 12wt) would never be used for a 20lb tarpon, nor would it be considered ethical. Is there any relation or function between grain weights and target fish size? Logically it sounds like it would be a linear relation, but fly fishing knowledge suggests otherwise.