I really don't understand why minimum css are used
for example form admin.css , admin.min.css is loaded
I think WordPress might minimize and concatenate scripts/styles in just the admin area by default. You can probably change some of those behaviors - if you really want to - by defining some constants in your wp-config file; though I'm not sure that it's necessary if it's not causing any problems.
I really want to know what's going on here .......
Some cool stuff to look at in this file: http://core.trac.wordpress.org/browser/trunk/wp-includes/script-loader.php
As the name indicates i thought .min.css files contain min amount of css and load faster
That is correct, but it's not because of less css, it's because of less of everything else.