larger shared buffers slows down cluster

This problem has been reported by a client.

Consider the following very small table test case:
   create table bar as select a,b,c,d,e from generate_series(1,2) a,   generate_series(3,4) b, generate_series( 5,6) c,
 generate_series(7,8) d, generate_series(9,10) e;   create index bar_a on bar(a);   create index bar_b on bar(b);
createindex bar_c on bar(c);   create index bar_d on bar(d);   create index bar_e on bar(e);   create unique index
bar_abcdeon bar(a,b,c,d,e);
 


Now running:
   cluster bar using bar_abcde;


appears to be very sensitive to the shared buffers setting. In an amazon 
very large memory instance (64GB) and PostgreSQL 9.1.4, I observed the 
following timings:

    Shared Buffers   Time      48Gb           2058ms       8Gb            372ms       1gb             67ms


Is this expected behaviour? If so, is there a good explanation? I'm not 
sure what other operations might be affected this way.

cheers

andrew