Earlier quoted context omitted.
In June 2011 they had 11 million articles across all Wikipedias. As of now they have 55 million articles across all Wikipedias. [0]. I'd say this is pretty good even without considering any increases to viewership or size of existing pages. [0] https://en.m.wikipedia.org/wiki/Wikipedia:Size_comparisons
There is no big differences for the database in these numbers. Wikipedia doesn't save lives here. They can have different shards for read only data and doesn't need to process millions of different signals every second. And worth to mention that site growth is predictable.
On big thing that changes as a site grows, and as it reaches more places internationally, is that it attracts more attacks. Stopping that costs lot.