far different size.

Size first. Power electronics don’t have the same miniaturization benefits (or expectations) for laptops as they do for phones. Some factor here is that there’s just a different convention for these two different types of devices.

larger bricks that have separate mains cable which plugs into the brick

There’s a manufacturing benefit having one circuit where you can just change the mains cable for length or country adaptor types. High probably the wall warts have the same ability, but it isn’t as obvious if the plastic parts are press fit.

Why is that so?

Some laptops could run off of 65 w, but only the very basic types. An Alienware m18 R2 gaming laptop has a 280 w adaptor and it isn’t close to the same size or price as a phone adaptor.

Same for phones. 65 w is pretty beefy for a phone charger. It will come down to the requirements of the device.

Generally speaking, a charger can have more amps than the phone or laptop requires and that’s usually okay. The voltage is what you have to be careful about. 5v is kind of a common electronics safe voltage. Good chance all these “universals” chargers are 5v.

Again, something like an Alienware m18 R2 is going to require more than 5v.

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

Good chance of that. Much easier to price fix on big warehouse type online marketplaces.