Reduce some chunk sizes

As these queries are still slow enough to be logged.

guix-data-service/model/derivation.scm

@@ -1423,7 +1423,7 @@ INNER JOIN derivation_outputs
  AND vals.output_name = derivation_outputs.name")))))
 
   (chunk-for-each! process-chunk
-                   500
+                   250
                    (list-copy derivation-ids)
                    (list-copy derivations)))
 
@@ -1808,7 +1808,7 @@ WHERE derivation_inputs.derivation_id IN ("
                   #f "querying for batch of ~A related derivation ids"
                   (length ids-chunk))
                (exec-query conn query)))))
-        (chunk! ids 1000))
+        (chunk! ids 500))
        <))
 
     (define (derivation-ids->missing-sources ids)