Ice floats because water molecules occupy more space in the solid state than in the liquid state, so that a given volume of ice is lighter than the same volume of water.
The way molecules fit together in ice crystals is not as compact as in water.
They don’t change size, it is just the way they are arranged.
Most substances have more tightly packed molecules in the solid state.
If this were true of ice, it would sink to the cold bottom of the ocean and never melt, so that the seas would be frozen nearly to the surface.
The word “ice” comes from the Old English word “ís”.