Later in the war, they used a different charge in the grenades, and the heads were reduced in size, but rather than using the old explosive or putting more in the old larger heads, they put a bit of wood in the larger heads to reduce the room for the explosive of greater power.
Bes twihses
Gus
ps It is very dangerous to handle the explosives in the older grenades, unless you know what they are, the Japanese grenades for example, often used picric acid, and it is very unstable after it has aged or come in contact with iron or steel