Holy handgrenade has a fixed explosive power of 300, while a mine's total explosive power depends on the volume of its load and the load's explosive power:
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Volume * MaterialExplosivePower / 1 000 000
A big mine has a load volume of 12500, and is normally filled with gunpowder with ExplosivePower = 15000. Therefore, a big mine has an explosive power of 188, or about 2/3 of a holy handgrenade.
Normal mine has a load volume of 5000, thus an explosive power of 75.
If a big mine was filled with neutronium, which has the highest explosive power in the game (ExplosivePower = 120000) but doesn't appear unless scripted in dungeon files, it would have an explosive power of 1500, or five times as powerful as a holy handgrenade.