Melted ice cream refrozen at home tastes icy. What changed most?

Answer: Bigger ice crystals

Bigger ice crystals ✓ — Right. Smooth ice cream depends on many tiny crystals that the tongue barely notices. Once the pint warms, the smallest crystals melt first; when it freezes again, that water tends to join existing larger crystals instead of making a fresh population of tiny ones. The surprise is that the total amount of ice may not change much, but the crystal size distribution does. Texture is a geometry problem as much as an ingredient problem.

Less dairy fat left — Not quite. Fat helps creaminess and foam structure, but a short melt-refreeze event usually does not remove dairy fat from the pint. The bigger damage is that ice rearranges: small crystals disappear and larger ones grow. A high-fat premium ice cream can still turn coarse after heat shock, which is why storage history can matter as much as recipe quality.

More frozen sugar syrup — Almost, but backwards. Sugar is part of the unfrozen syrup phase that keeps ice cream scoopable at freezer temperatures. Refreezing does not simply freeze more sugar syrup into a smooth block; it gives melted water a chance to feed larger crystals. That is why a sweet pint can still become gritty after warming.