Whether you're familiar with the ISS or CSS or not, watch this guy's video for a comparison of ISS, CSS and Mir.

So, I assume you know a bit about the ISS by now. I'm just going to highlight about a few points between ISS and CSS.

How Tianhe compare with the largest ISS modules. This diagram is not very accurate but gives you some idea.

Next, this is the Integrated Truss Structure (ITS) of the ISS. It contains four pair of large solar array wings.

If you remove the largest structure in the ISS, i.e. the ITS, the remaining modules for living and science on the ISS, aka the orbital segment, is not really very large compared to basic CSS with three modules. Especially so if you exclude the highly independent Russian segment. The ISS has many utility nodes to join the modules from the different countries, and airlocks for docking spacecraft. When you say different countries, there're actually only four. Out of the six permanent crew, two from Russia, two from US, one from Europe, one from Japan. In terms of lab module, Destiny from US, Zarya from Russia, Columbus from EU, Kibo from Japan. The Russian segment can fly away and survive, they've independent power production, dock for spacecraft rendezvous etc. The EU and Japan modules can't, they totally rely on US segment for power, transport, living etc.

The ISS orbital segment without the ITS doesn't appear large especially when compared with the extended CSS with six modules. That's because every single module on CSS is over 20 tons and 16 m.

The noteworthy points:

• The four pair of solar array on ITS generate about 130 kW of power for ISS. The two pair of solar arrays in the two space lab of CSS generate about 100 kW of power. The three module basic CSS generate about 80% power of the ISS, thus CSS doesn't need a very large structure for solar arrays.
• The three modules CSS support three crew. The extended six module CSS support six crew. The ISS has six permanent crew.
• The Destiny lab has 13 sciencetific racks, the Columbus has 10, the Kibo has 10. Not counting the Russian, the US segment on the ISS has 33 scientific payload rack compared to about 24 in the three module CSS, or 48 in the extended six module CSS. This space lab diagram illustrate how it support 12 experimental racks.
  
  

Here, you can see the actual 12 experimental payload rack mounted in a single module, presumably a space lab module.

In short, the CSS doesn't need to be as large, heavy, complicated, and costly as the ISS to have more power production, experimental capacity and support equivalent crew size in the extended version.