Kurt Russell's character Jack Burton was fully human in Big Trouble in Little China. Then why he is shown so large in movie poster?
Is it for marketing the film on the name or Kurt or there is some symbolic meaning behind this representation?
Adding to Paulie_D's answer and fleshing out cde's, the image is also a literal interpretation of the title (which is metaphorical).
So, considering Big Trouble in Little China, Kurt Russel (the big trouble) is shown to be so much larger in the image than everyone else (the little china).
Not only does this emphasize Kurt Russel's role as the main character (as Paulie_D mentioned), but it is also a smart/funny stylistic interpretation of the movie's title (as cde mentioned).