Guys,
Perhaps I can help out here.
The main drivers for weight difference between the C and the A are the bigger wing, tail and fins (all weigh more) and the massive amounts of extra metal needed internally to handle cat and trap loads. Landing gear weight is more than twice the A's. C's greater internal fuel capacity comes mainly from the bigger wing, but LO is quite right to point out that losing the very large gun installation helps as well. (So does losing the USAF refuelling receptacle - that eats up lots of lovely fuel tank space).
The gun installation is interesting - the original 'A' design used a Mauser 27, but under pressure from US domestic manufacturers (and concerns over Mauser feed issues), LM went for a 25mm gatling. Nice gun, but larger, heavier, less effective and needs more ammunition per target.
So what does the A do that the C doesn't? Main attributes are higher sustained G, faster acceleration and higher speed. It's also a lot cheaper, but building lots more will always do that for you.
Hope this helps a bit,
Best Regards as ever,
Engines