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