Th reason is quite simple.Although laptop power consumption is smaller.
And Adopt flyback topology circuit with conversion rate up to 85%.
However,Since laptop adapter is in a sealed plastic case.

It is very difficult to discharge heat,which result in the temperature keeping up directly.

Now,maybe some users will ask,why does the temperature of my laptop adapter is higher than others?
Well, the main reason is differnt laptop has different power consumption,

Meanwhile,the electric energy that adapter needs to provide is different. Although they are the same model,

the power consumption to operate the big game is much higher than it to operate the office software.

And now if you charge the battery again,the burden of power supply will reach the maximum.
And the fever will be very serious. So it is possible to reach up to 80degrees.

Please be careful to use the laptop when adapter becomes extremely hot.
When the weather is hot,please do not  cover anythings on the laptop.
And don't put the laptop adapter on the cotton quilt.

Otherwise,it will not only accelerate the aging of adapter,but also leads to damage
And even fire(under the extreme conditions).

Since sealed plastic case is the main cause of laptop heat,then why adapter adopt sealing structure? 
Why does the laptop not use metal case or open ventilation holes on the case?

The reason is simple.Because the power supply has 220V voltage.
Once using the matal case,if it comes down or any other reasons.

will cause short circuit between the case and electric supply.

Thereford,it leads to electric shock events.We don't open the ventilation holes on the case.
not only to make sure the strength of the case,but also avoid sundries such as the water,dust from outside.

After weighing the pros and cons,the adapters at present are still using the sealed plastic case,

and adopte ultrasonic welding or glue to seal up.

Why Power Adapter could burn down the laptop?

When you are going to buy the second adapter,it is usually hard to buy original ones.but the replacment adapter. 
Now the connector type,output voltage and output current become the replacement keywords.

The same connector is needed,and the voltage should be the same too.
If you could not find 100% the same with originals,but it would be fine if the errors are within 5%.However,

if the output voltage is very big different,the laptop will not work or even breakdown.

There are still some people think the current should also be coincide.
Because big current adapter could not replace the small current transformer.
Otherwise, it will be broken if the big current pass into the laptop.

Actually this is a misunderstanding, we could know clearly according to the Ohm's Law I=U/R.

When the voltage does not change,the current depends on the load resistance

Which means,The laptop working requirement decides the current. Not depends on the big output current.
So, the good adapter's calorific value will still be reduced if the big current adapter replace the small current adapter.

Attention please!Don't use small current adapter to replace big current adapter. 
Now lets take water pipe as am example,although The tap is opened to the largest,
its water yield (current) still can not meet the requirement of application.

At this time,the laptop will crash regardless or restart.

Why the weight is different?

In fact,it is obvious that different laptop adapters has different weight.
Even if eliminating the difference about the main material and heat emission because of different powers,
among the same power of original laptop adapters.

their weight have almost doubled different.Some third-party compatible adapters are lighter.

In this case,the laptop adapter should be heavy enough,to ensure the stability of laptop adapters.
"The easy people" think that,because the laptop becomes light and slim,adapters need to "ease the burden" too,

otherwise,it is really not easy to take. what a absolutely joke is that1KG laptop comes with 500G adapter.

It should be said,both of them are right.
But now, how should we see the light laptop adapters?Please see the two cases to lose the weight as below.

One case is The Essence of Concentration. 
Depending on the improvement of circuit and design.
the manufacturer use the higher power converted efficiency to decrease the weight of internal heat sink and element.
so as to reduced volume and weight.

Most of original laptop adapters and compatible laptop adapters are do like this,you may rest assured to buy it.

Another case is jerry-build,they usually decrease some proper protection(such as PFC circuit).
even cut down the material on heat sinks and case. 
The voltage of these adapters lack of stability,even the strength of case is really bad. 
Please,pay much attention to it when you plan to buy.

It is fine to take 19V4.74A adapter intead of 19V3.9A, you only to check if the connector is right.

Laptop Adapter is the leading role. Choosing one full power and good stability adapter is the basis of laptop.