I learned a ton about weight from Chris DeRoachie. Summed up it is that since everything falls at the same speed (according to Newton) then the length of time it takes for something to fall tells us how big it is. In other words, if a flour sack is jumping off a table, it will take the same amount of time as a pencil falling off a table (so drop a pencil off your desk). So if you want a giant to feel big, you have to think about how long it would take a pencil to fall from the top of his step to the ground, and put that time into his footstep. (so if the giant is stepping over a house, how long does it take for a pencil to fall from the roof.)
I later experienced this directly when animating a giant elephant man creature. I animated it falling down like a guy, because it filled up the screen like any old guy. Then my lead told me to slow it down by half. And then half again. And it felt much bigger.
The other day looking at Brendan Body's blog I learned something new to help me refine my understanding even more. He dropped a tennis ball from a bunch of different heights and noticed the longer it fell the more even the spacing became. So for a bigger creature he animates it slower with more even spacing. Check out his full tutorial on it.