The 22,000 lb Grand Slam had an explosive content of about 9000 lb. The very heavy casing was needed to allow it to penetrate deep underground before shattering the foundations of its target.
A standard Lanc with a bulged bomb bay could carry a 12,000 lb cookie, basically three of the 4k bombs we have now bolted together. Cookies were thin cased bombs which could carry 3/4 of their total weight as explosive. So a 12k cookie had a similar blast value as a Grand Slam, it just exploded on the surface.