flash size 1M ?

Hello,
I tried to flash a firmware on development kit basic through uart and I get the message : file xxx.bin is too big.
Size of my firmware is 630kB and flash is 1024kB, so I don't understand why it doesn't work.
I check in code of cli_programmer, and there is a condition on firmware size to be sure that firmware is smaller than 508kB.
Can you explain me why ?
Thanks,
Jérémy