Your question refers to a phrase in the opening paragraph of O. Henry's "The Gift of the Magi". The phrase refers to the method by which Della has been able to scrape together a small amount of money, in other words repeatedly haggling with the grocer, vegetable seller, and butcher. The type of figurative language used in this phrase is metaphor. A metaphor is a comparison of two things by stating or implying that one thing is some other thing. In this case, Della is compared to a bulldozer because the pressure with which she argues with vendors to lower their prices for her is apparently forceful enough to knock over their objections, just like a bulldozer knocks obstacles out of its way. "Bulldozing" is a much more vivid way to describe Della's haggling than to say that she had "argued" about the prices of things.