The Aygo, CI and 108 are built in the same factory - a Joint Venture set up between Toyota and PSA, and the factory is owned and operated by the Joint Venture. Toyota teamed with PSA due to their experience in Europe of building small cars, and to share the design and production costs. Whereas in Japan and Asia they rely on Daihatsu for their small cars (some Daihatsu models are rebadged as Toyotas) for similar reasons - although Daihatsu is now wholly owned by Toyota.
Profit margins are tighter on city cars than superminis, medium sized cars (Auris), etc, so parts will be designed to a meet a smaller budget than larger cars. For example most city cars have areas of painted metal visible in the interior and partial door cards, whereas this wouldn't be acceptable in medium sector cars.
If Toyota hadn't partnered with another manufacturer, we wouldn't have had a Toyota the size of the Aygo in Europe.