Installed MacPorts packages sizes
You could make use of port build in command contents, which gives you the oportunity to do things like that:

port contents --size depof:python27


Grep the lines you need and do the maths :-)

Update: found answer here at guide.macports.

While reading a bit deeper, I found this

port space --units MB --total thisport


which is much more handy then calculating the sum of the size of each file inside a port.