comet soda pop, if so much debris and dust falls off a comet why doesn'y it just get smaller and disappear?

In the article, "‘Soda pop’ comet has tons of sizzle", it states:
( [link to www.futurity.org]

“We estimate ISON is emitting about 2.2 million pounds of what is most likely carbon dioxide gas and about 120 million pounds of dust every day,” says Carey Lisse, leader of NASA’s Comet ISON Observation Campaign.

So, in one year or 365 days the comet will have emitted 43 billion 800 million pounds of "dust". (365 days X 120,000,000)

The article states that the size of Soda Pop: "Comet ISON (officially known as C/2012 S1) is less than 3 miles in diameter, about the size of a small mountain, and weighs between 7 billion and 7 trillion pounds. Because the comet is still very far away, its true size and density have not been determined accurately."

So, even at the higher end of the estimate of 7 trillion pounds, wouldn't the comet be "gone" after just 159 years. That is a concept I could never understand, if all of the debris is trailing the comet comes from the comet, why don't they eventually just disappear?