Some days, it seems as if every museum in America is mounting a major expansion, or at least talking about it. But at The Museum of Contemporary Art/Denver, the desire for more space is tempered by the realities of the marketplace, and the museum’s directors are anxious to do more with less. Rather than attempt a massive new building project, MOCAD is planning a modest expansion with a price tag of less than $4 million, which it hopes will generate buzz without endangering the institution financially, or alienating the public with demands for government subsidies in a notoriously conservative state.