How are gsp,jon fitch, and josh koscheck so good at takedowns.?

how are all these shrimpy ufc fighters so good at wrestling. example:gsp vs sean sherk. i mean i understand how dan henderson is good at takedowns cuz he uses clinch sweeps, but gsp,fitch, and koscheck use takedowns. how do they take huge guys like sean sherk,matt hughes,diego sanchez,and anyone else that is huge.