select_parallel test fails with nonstandard block size

When building with --with-blocksize=16, the select_parallel test fails
with this difference:

explain (costs off)
select sum(parallel_restricted(unique1)) from tenk1
group by(parallel_restricted(unique1));
- QUERY PLAN
-----------------------------------------------------
+ QUERY PLAN
+-------------------------------------------
HashAggregate
Group Key: parallel_restricted(unique1)
- -> Index Only Scan using tenk1_unique1 on tenk1
-(3 rows)
+ -> Gather
+ Workers Planned: 4
+ -> Parallel Seq Scan on tenk1
+(5 rows)

set force_parallel_mode=1;
explain (costs off)

We know that different block sizes cause some test failures, mainly
because of row ordering differences. But this looked a bit different.

The size of the tenk1 table is very similar under either block size:

16k: tenk1 = 2883584
8k: tenk1 = 2932736

Is there an explanation for this difference, or is there something wrong
in the cost estimation somewhere?

--
Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services