I like using the analogy of 'living in a small apartment' when building systems with a small team. You need to choose carefully what furniture you can fit into your apartment, and that choice depends a lot on how you live your live. Do you want a large table to host friends, or a comfortable couch to fall asleep on in front of the TV? If you get both the space will probably be cluttered.

The same applies to a small software project - you need to choose what features you can fit. And while the cost of building is part of the consideration, I'd say most of it is about the cost of maintaining features, not only in code, but also in product coherence and other incidental 'costs' like documentation and user support.

Be careful of building too many features and ending up being overwhelmed by the maintenance, or worse, diluting the product's value to a point where you loose users.