Re: Dozens injured in two explosions at Boston Marathon finish line
w/ no expertise, the guy in the picture on the last page doesn't appear to be holding anything terrible heavy. I don't know how much a pressure cooker filled w/ explossives and shrapnel weighs exactly, but that guy appears to be standing at ease holding his big in a position that suggests there is something light inside of it (perhaps someone's cloths, camera, car keys, etc... that he came to watch cross the finish line).
Now the guy (actually I'll say guys) in the 2nd picture at the top of this page is in a position where he could potentially be holding something of significant weight. That is exactly how you'd place your bag if you were sitting around w/ 20+ pounds in your bag. the guy that he's talking to in left side (or at least appears to be talking to) is also holding an over the shoulder bag in a position that you'd hold something of significant weight.
Of course that could just be the most comfortable position to hold the bag in both cases, and the bag could be pretty light. Whatever is in the black bag, it appears to be stuffed pretty full and is pushing something out the back.