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?
One significant difference that has been missed here is that Laptops can and often do run on the power supply, while phones usually use the power purely to charge the battery.
It’s a significant difference as the laptop needs a stable electricity supply to supply all it’s components or the laptop would crash. That means not only does the brick need to dissipate heat, but it also needs to be able to deliver a stable continuous DC current. So as well as a transformer and rectifier (that together convert AC to the correct DC needed) there are smoothers and potentially capacitors to ensure a smooth continuous output even if the wall supply is janky.
If you turn off the power at the wall / unplug you often see any light on the power brick stay on - that is because of the capacitors and there is still a small amount of energy available to the laptop as it discharges.
While phones are mini computers they are usually designed to always run on the battery. Even when charging, the phone draws it’s power from the battery and it’s in built circuits to smooth the current; there isn’t usually the redundancy in a phone to switch between different supplies in the same way as a laptop. There isn’t also the expectation that they need to run off the wall continuously by users (even if users might plug their phone in and expect to continue to use it, they will find their phone shuts down if its at 0% and they push it beyond what the recharging battery can supply; a laptop would be expected to run solely on the wall not shut off).
Things are blurring now with USB C power supplies for laptops - but you will find the plug itself has more of the electronics built in or some of the functions of the power brick have moved into the laptop to reduce charger bulkiness. Look at how bulky a USB c charger plug is for a Mac - they’re not simple USB chargers you’d use for a phone or tablet, they’re bulky because they are also doing the smoothing and stabilisation people expect for their laptops.
Tl:Dr they’re different sizes because they’re doing different things. Basic chargers purely charges a battery, while laptop chargers both charge a battery and provide good quality supply to keep a laptop running optimally.