It is likely that you have a 512k agnus installed. IIRC Agnus is mapped to control all memory access from $000000-$1FFFFF, and if you have an original 512k chip, the 68000 can't see the ram above 512k if its mapped into that address space.
On older machines, the memory above 512k was mapped to $C00000. This is what Kickstart understands, and it puts the memory into the proper (non-chip) memory pool.
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