"Sorry, but heavy bombs have shells that must be able to penetrate concrete or hardened steel before detonating."
Not so fast. Heavy bombs can be heavy because they're designed around a strong armour-piercing casing with relatively little explosive filling , but they can *also* be heavy because they're designed around a thin-wall casing to maximise the amount of explosive within and thus blast effect once dropped...
e.g. Tallboy, 12,000lb total mass, approx. 5000lb explosive content
Cookie, 12,000lb total mass (in its largest variant), approx. 9000lb explosive content
Two bombs with the same overall mass, but designed for two very different types of mission.