weight vs diameter

I will assume you are talking pellet weight and head size, if that is the case we also need to know what type of shooting we are talking about. My personal experience has been that in my rifles past and present that head size is more important than weight at distances shot in field target, (up to 55 yards) Using a pellet gauge you will find a particular head size that will shoot best and virtually eliminate flyers. Going by what size is marked on the tin is pretty much useless as they will vary a lot and not be what they say they are on the tin (at least with some of the JSB types anyway!! 177 10.34 being especially bad) For shooting long distance the weight becomes more of a factor and it will vary quite a bit with the larger pellets used in long range shooting, (.25 /.30 cal.) 

It still all boils down to how precise you want to be with your shooting , but I can assure you it will make a difference in MOST all cases!! Some barrels seem to shoot most anything you put in them very well but I can assure you with a bit of work you will find a size and particular brand /type of pellet that is head and shoulders above the others. Might take some work ,and it can be VERY boring and long and drawn out but to me very rewarding in the end!! Hope this is helpful to you, Good luck!! J.L.