There are propably two reasons for those giant builings:
Old school science fiction art, which is often optimistic, but tends to like large scale stuff, because bigger is better.
Architects selling their visison. It is reasonably common for architects to just design a dream building with unlimited budget and those tend to be massive. Right now there is a trend to stick plants on every part of a building, to make them appear more enviromentally firendly. That is usually not the case, but it does look cool.