vacuum analyze fails: "Tuple is too big"

============================================================================
                        POSTGRESQL BUG REPORT TEMPLATE
============================================================================


Your name        : Palle Girgensohn
Your email address    : girgen@partitur.se


System Configuration
---------------------
  Architecture (example: Intel Pentium)      :   Intel Pentium

  Operating System (example: Linux 2.0.26 ELF)     :   FreeBSD 3.3 Release

  PostgreSQL version (example: PostgreSQL-6.5.2):   PostgreSQL-6.5.2

  Compiler used (example:  gcc 2.8.0)        :   gcc-2.7.2.3


Please enter a FULL description of your problem:
------------------------------------------------
In getting around the fact that postgres limits the tuple size to 8192
bytes, I use a "bigtext" table that splits large text chunks into
smaller parts and store them in separate tuples:

Table    = bigtext
+------------------------------+-------------------------------+-------+
|              Field           |           Type                | Length|
+------------------------------+-------------------------------+-------+
| id                           | int4                          |     4 |
| seqid                        | int4                          |     4 |
| txt                          | text                          |   var |
+------------------------------+-------------------------------+-------+

When checking, none of the tuples is too big:
foobar=> select id,seqid,length("text") as len from bigtext order by len
desc;
  id|seqid| len
----+-----+----
9590|    0|6929
9593|    0|6899
9567|    0|6894
9567|    1|6892
9595|    0|6754
9592|    1|3003
9590|    1|2939
9593|    1|2693
9595|    1|1667
9567|    2| 293
(10 rows)

but still, vacuum analyze fails:

foobar=> vacuum analyze;
ERROR:  Tuple is too big: size 13700

normal vacuum is OK:

foobar=> vacuum;
VACUUM

When removing all tuples above size 6500:

foobar=> delete from bigtext where length("text")   > 6500;
DELETE 5
foobar=> vacuum analyze;
VACUUM

So, it works now... I thought the limit was 8192 bytes? seems more like
6500?

How come?

I know this 8192 tuple size limit can be raised when compiling, but this
is considered very risky, right?

Please describe a way to repeat the problem.   Please try to provide a
concise reproducible example, if at all possible:
----------------------------------------------------------------------
CREATE TABLE "bigtext" (
    "id" int4,
    "seqid" int4,
    "txt" text);

Populate it with some text chunks about 6800 bytes each in txt.

vacuum analyze