How speakers capacity can be measured?what we can observe from that?

The size of the speaker is a significant factor upto a point. if it is very small (as in the laptop) it cannot efficiently move the air - so low sound and missing bass (where a lot of the sense of the volume comes from) the larger external ones, can move more air.. Different speakers also have different efficiencies... I know of one set of 80 000 USD hifi speakers that generate 109dB for 1 watt in ! http://www.avantgarde-acoustic.com/trio-product-en.html (120+ will damage your ears permanently) and that will be extremely difficult to shout over - we are talking disco volumes.. (very nice sound by the way - not that I owned any, but....) whereas most laptop or even normal hifi speakers barely make 80 db for 1 watt in. and 2.5 watts is what a laptop will output at best without extra amps (powered external speakers will do more therefore)

Laptop battery or power supply has limited power, this is to make laptop as small and light as possible. Extra speakers have their own amplifier, and its only job is to power the speakers. To give an example: a laptop can use about 80 watts and it has to power first the computer and the screen, the laptop speaker probably only takes 2 watts. But an outside speaker system with its own power supply can produce maybe 200 watts of sound. A system like this is not really made to be small and portable like the laptop. You could have a small portable speaker system which is, say 20 watts, this is still much louder than the laptop speaker, but if you have to carry it and your laptop too, it's not so portable is it ?

Laptop battery or power supply has limited power, this is to make laptop as small and light as possible. Extra speakers have their own amplifier, and its only job is to power the speakers. To give an example: a laptop can use about 80 watts and it has to power first the computer and the screen, the laptop speaker probably only takes 2 watts. But an outside speaker system with its own power supply can produce maybe 200 watts of sound. A system like this is not really made to be small and portable like the laptop. You could have a small portable speaker system which is, say 20 watts, this is still much louder than the laptop speaker, but if you have to carry it and your laptop too, it's not so portable is it ?