How to donsize _DATA_GROUP_ ?

Dear all,
I have one question about the _DATA_GROUP_ in DATA MEMORY in m51 file.
The current DATA MEMORY is as follows:

REG     0000H     0008H     ABSOLUTE     "REG BANK 0"
DATA    0008H     000DH     UNIT         ?DT?A
DATA    0015H     0008H     UNIT         ?DT?B
IDATA   001DH     0001H     UNIT         _IDATA_GROUP_
        001EH     0002H                  *** GAP ***
DATA    0020H     0001H     BIT_ADDR     ?BA?A
BIT     0021H.0   0009H.5   UNIT         ?BI?A
BIT     002AH.5   0000H.3   UNIT         _BIT_GROUP_
DATA    002BH     0055H     UNIT         _DATA_GROUP_
IDATA   0080H     0060H     UNIT         ?ID?A
IDATA   00E0H     0001H     UNIT         ?STACK

_DATA_GROUP_ now is 55h bytes. But original size is 4Bh bytes before code merge.

The bigger _DATA_GROUP_ causes:
1. ?ID?A was put back and thus variables' offset was changed
2. The space for STACK was getting smaller...

I want to know how to downsize _DATA_GROUP_ and thus ?ID?A and ?STACK are not influenced ?

Could you point-out some detailed comments ?

Thanks in advance...

* I have tried to use some global variables to replace the local variables BUT _DATA_GROUP_ is still 55h in length...