MySQL 5.7 + Ubuntu 14.04 eating up my RAM
MySQL 5.7.13 is OK

I was suffering from this problem. Same as you described: MySQL 5.7.11 with almost empty database and few requests in hour was eating memory up to 450MB.

So I've recently tried new version and it is totally OK. MySQL 5.7.13 instance is running for a week already and it takes only 73MB.

Also I've turned off performance schema in my.cnf:

            [mysqld]
            performance_schema=0