Quality of material and the way that material is used. I can see Zommamel's point. I am going to go out on a limb here and assume that since the 1980s, better stronger plastics have been developed, which means that you should be able to use less for the same effect.
Sure, the hollow limbs look awful from behind and personally I'd prefer they were covered up. Let's say they did cover them up, we'd never know that he was hollow until you cracked him open.
TBH though, I don't really worry about the weight of Transformers anymore, although it is nice when you pick a figure out of the box, it's chock a block full of metal and it just has that heft to it. Although that can also be a bad thing. Alpha Trion's legs are generally too heavy to hold a set pose for example.