docs: Show the max savings of compressing JSON files

docs/GeoJSON-compression.md

@@ -18,4 +18,6 @@ Contra compression:
   - [AWS CLI issue](https://github.com/aws/aws-cli/issues/6765)
   - [boto3 issue](https://github.com/boto/botocore/issues/1255)
 - Any files on S3 "[smaller than 128 KB](https://aws.amazon.com/s3/pricing/)" (presumably actually 128 KiB) are treated as being 128 KB for pricing purposes, so there would be no price gain from compressing any files which are smaller than this
-- The extra development time to deal with compressing and decompressing would probably not offset the savings
+- The extra development time to deal with compressing and decompressing JSON files larger than 128 KB would not offset the savings:
+    - We can get the sizes of JSON files by running `aws s3api list-objects-v2 --bucket=nz-elevation --no-sign-request --query="Contents[?ends_with(Key, 'json')].Size"` and `aws s3api list-objects-v2 --bucket=nz-imagery --no-sign-request --query="Contents[?ends_with(Key, 'json')].Size"`
+    - Summing up the sizes of files larger than 128 KB we get a total of only _33 MB_ at time of writing