One bar of metal should absolutely be lighter than the ore required to make that metal. It should be lighter by a whole heck of a lot even. Iron ores are generally only around 70% iron, with the rest being various other materials. So you're removing 30% of the material when you smelt it.

Even if you were making steel, which would require adding a bit of material back in during the process, you're still going to be making an alloy that's 95-99.5% iron depending on the specific alloy. Your finish product is basically always going to be lighter than the ore required to make it.