revert undo file size

src/main.h

@@ -95,7 +95,7 @@ static const unsigned int MAX_TEMPFILE_SIZE = 0x1000000; // 16 MiB 0x8000000
 /** The pre-allocation chunk size for blk?????.dat files (since 0.8) */
 static const unsigned int BLOCKFILE_CHUNK_SIZE = 0x1000000; // 16 MiB
 /** The pre-allocation chunk size for rev?????.dat files (since 0.8) */
-static const unsigned int UNDOFILE_CHUNK_SIZE = 0x1000000; // 16 MiB
+static const unsigned int UNDOFILE_CHUNK_SIZE = 0x100000; // 1 MiB
 /** Maximum number of script-checking threads allowed */
 static const int MAX_SCRIPTCHECK_THREADS = 16;
 /** -par default (number of script-checking threads, 0 = auto) */