Several big rockets already were built - and all struggled, more or less, with lack of payloads. First was Saturn-5 - and N-1 arguably was closed because there wasn't enough interest in it, in terms of payloads, for anybody involved. Then it was Energia - again, may be, should USSR survive, it'd still flew, but it was closed, while Proton and Zenit survived. Last was Space Shuttle - which was particularly expensive lately with not enough missions to justify its annual cost.

So, with big rockets, it's doubtful that "build them, and they will be used" approach works. On the other hand, we really don't need big rockets that much anymore - we can do anything with existing rockets. It would be interesting to see how often Falcon 9 Heavy will be employed.