One other way to increase your allocated memory for FASTCGI is edit this file:
/etc/nginx/conf.d/fastcgi.conf
Find line:
fastcgi_cache_path /var/run/nginx-cache levels=1:2 keys_zone=WORDPRESS:100m inactive=60m
and adjust WORDPRESS:100m - where 100m is 100Mb according to your needs. But first, as I suggested above check how much memory to have allocated to /run.
Don't forget to restart nginx and php-fpm services after this change.
sudo service php5.6-fpm restart && sudo service nginx restart
or
sudo service php7.0-fpm restart && sudo service nginx restart
if you use php 7.