MySQL only for small places?
That makes no sense. Software licensing costs are always prohibitive at scale.
For a single machine it doesn't matter if you're adding 1k for the software or not. If you're doing that for 25 machines, it suddenly becomes a lot more important.
There's a bunch of larger websites around which have somewhere between tens and thousands of database servers around. Usually in a replicated setting which is very heavy on reads and has basically no writes which means they shard their databases in such a way that they fit into available memory and reads never go to disk.
In such a setting, your software being free is a very important point. Per server or per core licensing kills you there.
The usual option then is to go with MySQL or PostgreSQL. The latter has only relatively recently gotten acceptable replication so if you've been around a while you nearly always default to MySQL.
If you're at such a size, you either negotiate a very decent support contract or you forgo that anyway and hire the knowledge in house. I do have worked for a company which did both, I do know that at least Facebook has gone for the latter by hiring Domas. No clue what support contracts they do have. Same for Google.
If you're at that size, Monty will gladly listen to your needs and Percona will make you a very good deal for support.
Suddenly MySQL or MariaDB looks like a pretty great database with much better support options and costs than Pg or Oracle. Forget about MSSQL, you're not running anything on Windows at scale.