The PowerCenter clients are not small, they easily take up app. 2 GB.
Then you have the Developer client which on its own needs another 2 GB or so.
Finally there's a stand-alone installation of Eclipse (on which the Developer client is based), another 2 GB or so.
Not to forget all those ODBC drivers and quite a bit of supportive stuff, like the binaries for Mapping Assistant for Excel, Mapping Analyst for Visio, and the like.
Not to be forgotten, we also have the commandline utilities available as well when we install the client software.