I always see laptops come with larger bricks that have separate mains cable which plugs into the brick. Even if the adapter has some low power rating.
Example for comparison:
On the right is a standard laptop adapter.
Same price, same connector, same protocol, same power rating, far different size.
Why is that so?
Actually, the GaN adapter on the left also advertises itself as being meant for laptops, but by default, almost all laptops will come with something like the one on the right.
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 plug is integrated into it, which means the seller has to stock physically separate units for each plug type if they want to ship worldwide.
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.
Heat soak. A laptop may be expected to run 24/7. The larger bricks can dissipate the heat. A GaN charger can do it as well, but most of them have a heat soak point where they stop delivering full power