Why are my index files are much larger than my datafiles

-rw-------   1 postgres postgres       983040 Apr 26 20:33
si_tmpverifyaccountbalances
-rw-------   1 postgres postgres     18137088 Apr 20 13:03
si_tmpverifyaccountbalances_pke
=> select count(*) from si_tmpverifyaccountbalances;
count
-----
16176

w-------   1 postgres postgres       0 Apr 20 13:04
verifyaccountbalanceerrors
-rw-------   1 postgres postgres       516096 Apr 20 13:04
verifyaccountbalanceerrors_pkey
=> select count(*) from verifyaccountbalanceerrors;
count
-----
    0


The first index is three int4 values.  The second index is one int4 value.
Why are the index files so large?  What can I do to reclaim the disk space.
This is after a vacuum.  PostgreSQL 6.5 as of 4/25/99.   Red Hat 5.1 intel
pentium II.