i don't see why the weapons should be balanced against each other. railguns are expensive to research, but they have the best damage to size ratio, and they're also the least expensive to put on your ships. it makes sense it'd be the most expensive to research.
but i'm not here to defend mass drivers. i use the weapons that are best suited for the situation i'm in. a weapon's balance isn't in a vacuum, but also must be compared to appropriate defenses, their size/absorbtion/cost/research ratios. and ultimately, the best weapon is the one your enemies can't defend against.
i think you'll need a 9-dimensional graph

and that doesn't even factor in the sizemod values.
still, an interesting pair of graphs.