I think presently a chest plate takes about 3 bars, and most furniture (including statues) takes 3 bars. If it's hollow this sort of makes sense, and depending on what size barrels and bins actually are it makes sense enough for them too.
You can either have:
Iron Chest Plate: Weight 1,178
Iron Gauntlets: Weight 196*2
Iron Helm: Weight 157
Iron Boots: Weight 157*2
Iron Greaves: Weight 471
Iron Shield: Weight 393
Total Weight 2,905
Or:
9 Iron Bars, Weight 1570*9 = 14,130
Or:
3 Iron Statues, Weight 2,355*3 =7,065
Judging by the efficiency of the processes, forges don't melt metal down and reshape it into the required shapes, they take a solid block of metal of the size they want and chisel away at it until is the right shape and dump all the excess metal into a matter annihilator.
(* I haven't actually watched to make sure this is so, I just know when I process certain ore my smelters start producing multiple metal types)
Smelters don't produce partial bars when smelting ore, but some ores have a percentage chance to produce secondary products.