Compexity & Scope
Complexity and Scope I believe are what determines how big a project really is. However, there are several intangibles that can effect the size of a project as well.
The biggest downfall I faced was lack of requirements. In my particular situation the sales manager was determining requirements. His focus was on the sale... gotta get the sale. In his mind what the customer was asking for didn't seem all that complicated because we had built something similar. Vague requirements lead to underpriced jobs and over committed expectations.
Lack of a CCMU
CCMU is what I call a "Coo Ca Moo" (Clear Complete Mutual Understanding). You need to have a CCMU with your customer.
If you have a small project with a poor CCMU then you can wind up doing the project 2,3,4 or more times. Thus a simple 20 hour job turns into a 60 hour project with a stressed out staff and a very dissatisfied customer.
This happens more often than you think. The customer decides that since you are already doing A, B & C it shouldn't be that difficult to add D or even F. If this behavior is not checked it can also turn a small project into a medium size project. And depending on how the sales manager sold the job these scope creep expectations may seem "FREE" to the customer.