memoryjava.lang.OutOfMemoryError related with number of reducer?

I can fix this by changing heap size.
But what confuse me is that when i change the reducer number from 24 to 84, there's no this error.

Any insight on this?

Thanks
Lei
Failed to merge in memoryjava.lang.OutOfMemoryError: Java heap space
at java.util.Arrays.copyOf(Arrays.java:2786)
at java.io.ByteArrayOutputStream.write(ByteArrayOutputStream.java:94)
at java.io.DataOutputStream.write(DataOutputStream.java:90)
at java.io.DataOutputStream.writeUTF(DataOutputStream.java:384)
at java.io.DataOutputStream.writeUTF(DataOutputStream.java:306)
at org.apache.pig.data.utils.SedesHelper.writeChararray(SedesHelper.java:66)
at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:543)
at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:435)
at org.apache.pig.data.utils.SedesHelper.writeGenericTuple(SedesHelper.java:135)
at org.apache.pig.data.BinInterSedes.writeTuple(BinInterSedes.java:613)
at org.apache.pig.data.BinInterSedes.writeBag(BinInterSedes.java:604)
at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:447)
at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:435)
at org.apache.pig.data.utils.SedesHelper.writeGenericTuple(SedesHelper.java:135)
at org.apache.pig.data.BinInterSedes.writeTuple(BinInterSedes.java:613)
at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:443)
at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:435)
at org.apache.pig.data.utils.SedesHelper.writeGenericTuple(SedesHelper.java:135)
at org.apache.pig.data.BinInterSedes.writeTuple(BinInterSedes.java:613)
at org.apache.pig.data.BinInterSedes.writeDatum(BinInterSedes.java:443)
at org.apache.pig.data.BinSedesTuple.write(BinSedesTuple.java:41)
at org.apache.pig.impl.io.PigNullableWritable.write(PigNullableWritable.java:123)
at org.apache.hadoop.io.serializer.WritableSerialization$WritableSerializer.serialize(WritableSerialization.java:100)
at org.apache.hadoop.io.serializer.WritableSerialization$WritableSerializer.serialize(WritableSerialization.java:84)
at org.apache.hadoop.mapred.IFile$Writer.append(IFile.java:188)
at org.apache.hadoop.mapred.Task$CombineOutputCollector.collect(Task.java:1145)
at org.apache.hadoop.mapred.Task$NewCombinerRunner$OutputConverter.write(Task.java:1456)
at org.apache.hadoop.mapreduce.task.TaskInputOutputContextImpl.write(TaskInputOutputContextImpl.java:85)
at org.apache.hadoop.mapreduce.lib.reduce.WrappedReducer$Context.write(WrappedReducer.java:99)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigCombiner$Combine.processOnePackageOutput(PigCombiner.java:201)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigCombiner$Combine.reduce(PigCombiner.java:163)
at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigCombiner$Combine.reduce(PigCombiner.java:51)

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