The bitstream file size increases even with a small modification.

The bitstream comprises both fuse data frames (SRAM configuration bits) and EBR initialization information. Fuse data frame size is fixed for the given device irrespective of how huge is the logic. But the EBR configuration information size can vary; this depends on the number of EBR used.

In customer case, the portion of the code customer is commenting is DP_RAM, which uses EBR for its implementation. So, with DP_RAM enabled, the bitstream increases, because in such case the bitstream has EXTRA EBRs initialization information as compared to without DP_RAM.

In the example bitstream files (converted these bitstream into *.txt files for analysis purpose). Line 5669 of both text files show that fuse data frame size is same in both cases.
EBR initialization information starts from line 5676, the one with DP_RAM enabled has more EBRs this is the reason for variation in size of the bitstream.
The calculation below illustrates this:
This DP_RAM uses 3 extra EBRs, and each EBR size is 16 Kbits.
= ((16 * 1024) / 8) *3 -- gives the total number of bytes used by the DP_RAM
= 6144 Bytes
= (6144 /1024) -- to convert to Kbytes
= 6 KB

So, if we add this value to the bitstream size with the one DP_RAM disabled (2407 KBs), we get approximately 2414KB.