Describe Jim's appearance.

After Della sells her hair and uses the money to purchase a beautiful watch chain for her husband, Jim, he returns home. The narrator tell us,

He looked thin and very serious. Poor fellow, he was only twenty-two—and to be burdened with a family! He needed a new overcoat and he was without gloves.

In other words, then, Jim is rather slight. His physical thinness is, perhaps, a symptom of his salary being cut from $30 to $20 per week (which we learned early in the story). He also appears to be very serious, despite his relatively young age. This is likely also linked to his reduced salary, as this has clearly taxed his household (and his wife), requiring a degree of parsimony they never experienced when he earned more. The fact that Jim has a wife to support—a responsibility to her—seems to have taken its toll on him. In addition, important articles of winter clothing are either in bad shape or are missing completely: Jim seems to appear the way Della felt at the beginning of the story.

In "The Gift of the Magi" O. Henry doesn't describe Jim's appearance in that much detail, certainly not by comparison with his wife, Della. However, we are given some idea of what he looks like when he returns home after work on Christmas Eve. As Jim walks through the door he's described as being very thin. This could well be significant—a suggestion that perhaps he doesn't get enough to eat, what with his only making $20 a week.
It's notable that he isn't smiling, either. This could also be related to his low-paid job. Here's a young man of only twenty-two, and he has a family to take care of; the weight of the world appears to be on his shoulders. And those shoulders of his need to have a new coat on them, as the one he's wearing is way too old. From the scant information we're given, it's reasonable to infer that Jim appears prematurely aged by the privations that he and Della are forced to endure on a daily basis.