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?
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No USB power plug operates at 5 V when providing 65 Watts. That would be 13 Amps. Both power plugs most likey use 15 or 20 volts internally. 19 volts is not allowed over USB.
Edit: Here is a short summary of the USB-PD spec from Wikipedia
Volts are not a unit of power. Watts are a unit of power.
The 5V applies only to 4-pin type-A and type-B connectors.
USB Power Delivery over a regular USB-C-compliant cable delivers a variable current of up to 3A, using discrete voltages of 5V for up to 15W of power, 9V for up to 27W, 12V for up to 48W, and 20V for up to 60W.
Higher powers require dedicated USB PD-compliant cables that can handle up to 240W at a voltage of 48V.