For those who come after. I have found a solution to the issue. after digging through the files, I discovered the st-machine-flashlayout-stm32mp.inc file.
By modifying :
# 4GiB for sdcard
FLASHLAYOUT_PARTITION_SIZE:nor-sdcard:${STM32MP_ROOTFS_LABEL} = "4194304"
FLASHLAYOUT_PARTITION_SIZE:nor-nor-sdcard:${STM32MP_ROOTFS_LABEL} = "4194304"
FLASHLAYOUT_PARTITION_SIZE:nor-emmc:${STM32MP_ROOTFS_LABEL} = "3145728"
FLASHLAYOUT_PARTITION_SIZE:nor-nor-emmc:${STM32MP_ROOTFS_LABEL} = "3145728"
FLASHLAYOUT_PARTITION_SIZE:sdcard:${STM32MP_ROOTFS_LABEL} = "4194304"
FLASHLAYOUT_PARTITION_SIZE:sdcard-EFI:${STM32MP_ROOTFS_LABEL} = "4194304"
FLASHLAYOUT_PARTITION_SIZE:nand-4-256-1024-sdcard:${STM32MP_ROOTFS_LABEL} = "4194304"
FLASHLAYOUT_PARTITION_SIZE:nand-2-128-256-sdcard:${STM32MP_ROOTFS_LABEL} = "4194304"
FLASHLAYOUT_PARTITION_SIZE:nand-4-256-512-sdcard:${STM32MP_ROOTFS_LABEL} = "4194304"
Into :
# 4GiB for sdcard
FLASHLAYOUT_PARTITION_SIZE:nor-sdcard:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:nor-nor-sdcard:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:nor-emmc:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:nor-nor-emmc:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:sdcard:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:sdcard-EFI:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:nand-4-256-1024-sdcard:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:nand-2-128-256-sdcard:${STM32MP_ROOTFS_LABEL} = "12582912"
FLASHLAYOUT_PARTITION_SIZE:nand-4-256-512-sdcard:${STM32MP_ROOTFS_LABEL} = "12582912"
It solved all my issues. It would be nice to write it in the documentation because it took me a lot of time to find it. I now search a way to changes theses settings without changing them directly in the ST-layer but with an extern layer.