As far as the differences in blast radius go, judge for yourselves:
Take up B29's using 1 of three configurations: the 1k, 2k, and 4 k bombs. For starters, simply make a pass over the main area of town (roughly 2 to 8 O'Clock), drop 2 1k bombs together and film it. Watch the "spread" of the dual 1k bombs, then SINGLE 2k bombs, and then SINGLE 4k bombs. I've seen the 4k bombs reach out and grab auto ack further than one would think.
Remember this: you can drop all the 1k bombs you want and maybe have a "longer" blast radius due to the number of bombs being dropped, but the width of the blast radius is the *same*. If a player wants massive damage in a single pass then larger bombs are the way to go. Take notice how *wide* the damage is when a pass is made by the B29 using the 4k bombs. They can be dropped further apart and get get just the same amount of devastation. Remember as well that quantity does not mean "better" in terms of blast radius. There is no substitute for mass. This can be seen by dropping 1 each size of bomb on the city center, notice how far they all reach. Two 1k bombs dropped .05 seconds apart do not equal a single 2k bomb. Same goes for 2 500lb bombs vs a single 1k bomb.