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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