The season one, episode 15, 02:30 why the audience laugh? Please answer me if you know the reason,Thanks

Chandler quit his job , and Joey asked Chandler,
does that mean we have to start buying our own toilet paper?
and then ,Ross said ,That was Joey Tribbiani with the big picture , Dan ? I can't understand this sentence,because I'm from China ,does anyone know the reason why audience laugh when they heard what Ross said ?