Reduce the row group size

[iandees]

Continues #394

The builds are failing now because of memory problems:

<--- Last few GCs --->
[1:0x7f24ff58d000] 29254311 ms: Mark-Compact 932.6 (970.2) -> 927.6 (972.9) MB, pooled: 0 MB, 1000.28 / 0.00 ms  (average mu = 0.192, current mu = 0.032) allocation failure; scavenge might not succeed
[1:0x7f24ff58d000] 29255615 ms: Mark-Compact 935.4 (972.9) -> 930.3 (975.9) MB, pooled: 0 MB, 1287.91 / 0.00 ms  (average mu = 0.098, current mu = 0.012) allocation failure; scavenge might not succeed
<--- JS stacktrace --->
FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory
----- Native stack trace -----

This is an attempt to reduce the memory usage.