Scala keySet keep reference on self, forbig GC

CountSetMemory.scala

package test

import net.sourceforge.sizeof.SizeOf

object CountSetMemory {
  def bigArray = new Array(1024*1024)
  def main(args: Array[String]): Unit = {
    val map = Map(1 -> bigArray, 2 -> bigArray, 3 -> bigArray)
    val keys1 = map.keySet
    val keys2 = map.keySet.toSeq.toSet
    val keys3 = map.keysIterator.toSet
    // keys1: 12.003791809082031 Mb
    println("keys1: " ++ SizeOf.humanReadable(SizeOf.deepSizeOf(keys1)))
    //keys2: 3.8046875 Kb
    println("keys2: " ++ SizeOf.humanReadable(SizeOf.deepSizeOf(keys2)))
    //keys3: 3.8046875Kb
    println("keys3: " ++ SizeOf.humanReadable(SizeOf.deepSizeOf(keys3)))    
  }
}