That's not much of a difference anyway, if the buck were 6kg I would wonder about that myself.
But I cannot remember to have ever heard of someone actually having problems due to size difference, but quite often that it did work just fine. It sounds like a plausible theory, but on the other hand kit sized is determined by what the doe can deliver nutrient wise.
Maybe someone can chime in who has anectodal evidence of this being an actual problem, but with the weight difference you posted, even if it gets somewhat more, I wouldn't worry.