Well here is the thing, bombs dont explode on shield unless you get it on the shielded hitbox of the opponent, so in PM its technically a harder tech to do than melee. As for the size of the activation hit box is not the issue here, there are only a few things you could do to make this tech viable, which would be increase the amount of shield stun when both the bomb and its explosion hit on shield, so it gives samus more time to wd back to the center of their mass and bomb again, another option to change would be to just simply shorten the time it is required for a bomb to come out, so we could get 2 bombs out in the same time as one when they do explode on shield, so it doubles the hit stun the opponent is in and gives samus time to wd back to ground into another option for safe pressure