The Allwinner BROM is about 32KB, almost completely reverse-engineered at least for the earlier chips, and in any case the only thing it does during a normal boot is to load a chunk of code from a fixed location on the SD card to a fixed location in SRAM and immediately jump to it. It's nothing the Raspberry Pi boot process, where even the initial boot ROM has FAT filesystem support and it loads a proprietary blob which runs continually on the "GPU" co-processor with full access to RAM handling various important runtime functionality.