Index storage size growth on reindex


I am using an elasticsearch cluster in version 6.2.2 for gathering and analyzing logs.
The stack also includes Logstash and Kibana, both in version 6.2.2.

The index mappings underwent some rework and resulted in a new template.
Like documented, I used the reindex API to apply the mapping.

What baffles me was growth in size.
The original index which was created with a dynamic mapping takes 6.9 GB disk space. The reindexed one which uses the template takes 15.4 GB.

Changes in mapping look mostly like this:

"ApplicationTime": {
   "type": "text",
   "fields": {
      "keyword": {
         "type": "keyword",
         "ignore_above": 256


"ApplicationTime": {
   "type": "integer"

So I mostly changed to a single integer or keyword type.

How come the difference in size although the mapping is "smaller"?

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