Is it intended that big rocks only drop stone if destroyed?

Not intended. Do you know the internal name of the type of rock? I may have missed one type?

Alien biome rocks. There's a whole load of them.
might be easier to write a script that logs all rocks and their loot.

This is how I'm currently doing it, and it seems to match all the alien biomes rocks I can find in game.

for _, e in pairs(data.raw["simple-entity"]) do
  if (e.subgroup == "grass" or e.subgroup == "wrecks") and e.name:match("rock%-") then
    local adjusted_max = e.name:match("huge") and max * 2 or max
    local adjusted_min = e.name:match("huge") and min * 2 or min
    util.add_minable_result("simple-entity", e.name, {name="zircon", amount_min=adjusted_min, amount_max=adjusted_max})
  end
end

If you can give me an example internal name I'll investigate further.