Then you end up having deep expertise in a topic that's only important for larger companies. Sometimes this works out well, sometimes it leaves you a little stuck. :)
Meh. Just learn, learn, learn. If it isn't completely applicable to your life that's okay. Sometimes there are pearls of wisdom in best practices in field completely unrelated to your own. Learning something new is always a good thing.
THIS ^^^ So much this. I can't tell you how many times I picked up a book or paper thinking "There's no way i'm going to get anything new out of this". As I start reading, I start to find little tidbits of information that make me think in ways I didn't before.
Oh, there's plenty of demand for skilled DBAs with deep expertise with handling large volumes of data, even at small-to-medium-sized companies. GitLab being one of those companies! In fact, demand for this sort of thing is likely to increase. If it interests you, dig in. You'll never stop learning.
I'm not sure if it's really company size, though there's probably a correlation. My company is only marginally bigger than GitLab in terms of employees (just recently 200+), and we have several dozen databases bigger than this.
This, it depends more on the product and the requirements. I've worked in companies that are just a few employees and are running databases larger than this. I've also worked for a company with ~100 employees whose only 'database' is an in memory cache and a few kilobytes big textfile