For a small or weak site the line falls firmly on the side of better pages, and it holds there until you have enough coverage to actually compete in your topic. The boundary is coverage-sufficiency measured against quality-per-page: while your coverage is thin and your domain unproven, every page has to earn its keep, so you lean to quality first and only add breadth once each new page can pull its own weight. Volume early is not growth, it is dilution.
The reason is what thin volume does on a weak domain. A site with little accumulated trust gets a limited budget of attention from search engines, and spending it on a pile of shallow pages triggers the index-and-quality problem: pages that crawl slowly or never index, and a sitewide signal that this is a low-effort property. Each weak page you publish makes the average worse and gives the domain one more reason to be judged thin. A handful of genuinely strong pages does the opposite, concentrating your limited trust where it can convert into rankings.
The line moves as you cross it. Once you have real, defensible coverage of your core topic, pages that index and rank, a domain that has earned some standing, then breadth starts to compound rather than dilute, because new pages join an established base instead of stretching a thin one. At that point adding more pages is the right next move, but the test for each one does not relax: it still has to be strong enough to earn its place, not just exist to inflate the count.
So if your site is small or weak, prioritize fewer strong pages over more weak ones, and judge readiness for breadth by coverage, not by ambition. Build out your core topic to the point where it genuinely competes, treat that as the line, and only then begin adding pages, each held to the same quality bar that got you across.