Or is it simply cheaper to manufacture while being sold for the same price?

Yes, it’s mainly that. Most vendors don’t make their own power adaptors anyway, since there are strict requirements for power adaptors that are plugged into AC wall sockets in each country. The stuff inside the large brick is likely sourced from somewhere else, who has all those certifications worldwide. Note that it also uses standard AC power cable plugs. That large brick can be shipped worldwide by simply changing the AC cable it ships with.

The small GaN Brick is also outsourced, but since it is smaller it is probably more expensive to build. However, the AC plus is integrated into it, which means the seller has to stock physically separate units for each plug type.

As far as the price is concerned, this one shop is selling them all at around the same price, but I bet the MSRP on the Lenovo one is much higher, because of the brand. The parts inside the Lenovo one are probably cheaper.