BIOS/GPT with bios_gub raw partition: core image is too big

[moo can]

Hello,
I need to install grub2 on a x86_64 computer with BIOS and GPT partition 
table.If I understand well in BIOS/GPT, the bios_gub raw partition is here to 
host/handle the core.img (not writed in mbr).
So I made a bios_gub raw partition big enough to host the core.img:Number       
Start (sector)      End (sector)      Size            Code      flags1          
         2048                    12287               5.0MiB       EF02       
bios_gub
When I use grub-mkstandalone to generate the core.img 
grub-mkstandalone -O i386-pc -o core.img --modules= .... 

I have : core image is too big (0x185161 > 0x78000)
Is it possible to "bypass" this size limitation or is it hardcoded in i386-pc 
format even if the grub_bios raw partition exist ?
Thanks you for your help and advices.
Regards,Fabien