Why to think just big enough on inception ?

Most or almost all projects/products look limited in scope at inception. Slowly and steadily ground reality becomes clear. And comes the stage when it is required to reanalyze, redesign, refactor and start complete makeover of architecture. In simple words, how to limit the scope, how to define boundaries, how much variations actually need encapsulation, how many indirections really solves some non existent problems. Depends how big one can think. Think Big in pragmatic manner !!