src/Pure/Tools/build_history.scala

 {
   /* build_history */
 
   private val default_rev = "tip"
   private val default_threads = 1
-  private val default_heap = 500
+  private val default_heap = 1000
   private val default_isabelle_identifier = "build_history"
 
   def build_history(
     hg: Mercurial.Repository,
     rev: String = default_rev,