I didn't see anywhere that you were adjusting the size of boom_boom. You were setting the scale, but scale is non relative: if you keep setting the scale to the 0.5 every tick, that's not going to halve the size of the object; it'll just set it to half the size once.
Also, you had a trigger combined with 'once every 0.01 seconds'; in this case, you need neither. Replaced it with a simple 'every tick'.
I added the fade behavior to boom-boom. Deleted 'on created: wait x secs then destroy'. Fade has a built in 'destroy' command that runs when it's done fading. Great for explosions.
The boom-boom sprite was spawning way too big. Set it to spawn at 40,40 size.
Also, you were sending your walls/block off at random angles. No. Take a look at the modified events 7 and 9 on the Bubble_gum_bomb sheet. Neat trick: if you remove the 'random(170,190)' part, the walls/blocks will suck INTO the explosion.
I also did something else I don't remember. Keep an eye out for it.
I tweaked some of your settings -- timescale, etc -- to make my example clearer. Here's the capx:
Finally, you need more boo...erm bobs. WAY more bobs. We should be mowing down these insidious little malcontents by the bucketful. physics enabled bobs?
P.S.: I simply cannot resist:
. . .
DOLE KNOWS!!! <img src="smileys/smiley11.gif" border="0" align="middle" />