The following bug has been logged on the website:
Bug reference: 15066
Logged by: Marcin Piotrowski
Email address: mcinp@o2.pl
PostgreSQL version: 9.6.7
Operating system: Ubuntu 16.04
Description:
TL;DR
Gist index goes really huge when updating records, even if gist indexed
column is not changed.
Full story:
I already created a ticket for this, but was redirected to PostGIS.
Now it seems it's not PostGIS, but PG itself, as shown in
https://trac.osgeo.org/postgis/ticket/3716