Diaper genie 2 trash bag question?

When replacing a new bag do you pill the trash bag down until u cant ne more or do you just pull it as long as the genie itsself is? And also, I think I did it wrong the first time.. it says it can hold up to 180 diapers.. I'd say theres about 70 in there and I empied it and MAN was it heavy!!! So are you supposed to empty it every 30 diapers or so, cut the back throw it out and then instead of replacing the whole diaper trash bag just pull it down some more and tie it again? That makes more sence.. what do you do?