src/main/resources/builtin/loot/entities/slime.json

{
  "items": [
    {
      "default": {
        "item": "slime_ball",
        "count": 0
      },
      "conditions": [
        {
          "condition": "net.glowstone.util.loot.LootingUtil.isNot($.getSize(), 0)",
          "count": {
            "max": 2
          }
        }
      ]
    }
  ],
  "experience": {
    "count": "$.getSize()"
  }
}