CLUSTER using more memory than expected

The CLUSTER function seems to be using more memory than I expect.  Here is what I get from top and from my config file:

from top:
27589 postgres  25   0 3943m 3.6g  11m R 99.9 61.6 639:19.41 postgres: postgres data 127.0.0.1(42126) CLUSTER

from postgresql.conf
shared_buffers = 1000 #26214            # min 16, at least max_connections*2, 8KB each
work_mem = 256000               # min 64, size in KB
maintenance_work_mem = 2097151  # min 1024, size in KB
#max_stack_depth = 2048         # min 100, size in KB

My version is: 8.0.3

Any ideas why this would be happening.  I tried a full restart and the same thing happens.

Thanks,

Josh


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