If they are measuring plasma concentrations, the more plasma in a body, the lower the concentration for the same dose. Blood volume is directly proportional to weight.

All this data aside, what matters is how the person reacts to a specific dose of gabapentin. All the theory in the world will not over-rule how an individual responds.

If plasma concentration peaks at 3 hours, I must be responding to a lower concentration. Maybe the higher concentration achieved later is what helps me sleep so soundly.