Restore unequal min and max default heap

After considerable discussion, we have elected to set the default min
heap to 256m and the default max heap to 2g. This is to balance the
desire for a good out-of-the-box performance experience (default max
heap of 2g) with a good out-of-the-box experience running on machines
with limited resources or running multiple instances on a single modern
developer laptop (default min heap of 1g).

distribution/build.gradle

@@ -442,7 +442,6 @@ task run(type: RunTask) {}
  * </dl>
  */
 Map<String, String> expansionsForDistribution(distributionType) {
-  String heapSize = '2g'
   String footer = "# Built for ${project.name}-${project.version} " +
       "(${distributionType})"
   Map<String, Object> expansions = [
@@ -462,8 +461,8 @@ Map<String, String> expansionsForDistribution(distributionType) {
       'def': '',
     ],
 
-    'heap.min': "${heapSize}",
-    'heap.max': "${heapSize}",
+    'heap.min': "256m",
+    'heap.max': "2g",
 
     'stopping.timeout': [
       'rpm': 86400,