An early-stage company really needs two things from its customers: cash flow and feedback. It's fine if you don't make any profits, but you need revenue coming in the door to get future rounds of funding. It's fine if your revenue per customer is lower than you would like. Small customers are great at both: price it low enough to start that it's not a huge business expense for them, and you will be able to deal directly with the people who will use the system. If they have a problem, your product guys will know and be able to relay it to development.
Big customers often get you neither. They require lengthy contract negotiations that will rack up thousands in lawyer's fees from your side before you bring in any revenue. They ask for features that aren't core to your product and are likely years down on your roadmap (think LDAP authentication, customer admin functionality, audit logs, etc.) Again, these have to be developed before they sign a contract and before you see money. If you actually manage to get them into a production environment, they're much less likely to give you actionable feedback.
tl;dr: B2B software startups that target big contracts are much more likely to fail because the market dynamics of chasing after big customers will increase your burn rate while diverting your sales team's attention from smaller fish.