I stress that I am not Simon Schama or Robert Hughes. I honestly have no idea what Goya had in mind when he painted this, nor whether he intended the giant in the picture to be literal or allegorical or what. What I can tell you though is what I get from this picture (because, Goddammit, this is my blog and I'll post what I want).
When I look at this picture, the Giant seems to me to be the epitome of sadness. You get some idea of the scale which is intended from the tiny cluster of houses and a church in the mid-foreground (they don't really show up on the jpeg version I'm afraid). Given both the scale and physique of the Giant, it's clear it is the epitome of invulnerability: the concepts on offer in the picture give no reason for thinking its physical security could ever be endangered. But (and this is where the sadness appears) this strength seems to be more of a curse than a blessing. Its sheer size means it will be terrifying to any community it encounters. Whatever it might do to interact with a person or thing - by way of physical contact or even communication - will likely destroy them. And there is no sign of another being of a similar nature to share its sadness.
In effect, the Giant seems to be an entity which has acquired all the traits of a God except the capacity to interact with Creation save to destroy it. One thinks, perhaps, of the words attributed (almost certainly falsely) to Alexander the Great:
Which may explain the body language in the picture: the slumped shoulders of lassitude and utter boredom, and the head turning, desperately hoping to hear something intended for its own ears.
Perhaps this is the saddest thing imaginable: Having achieved the ne plus ultra in invulnerability, it lies upon him like a curse which cannot be lifted. One looks, and finds no exit.