For anyone who is doubting that assertion, it’s all about the costs that scale with the number of customers you have. Beyond the hosting and server costs, there’s support costs. There’s customer acquisition costs (eg click-throughs on ads scale with the number of people click the ads). For more expensive/enterprise SaaS, there’s the account managers and sales team.

You can grow your customer base dramatically with a smallish team of engineers/recurring engineering costs, but many of the costs surrounding them still grow in a linear way.