Jim seems so larger-than-life to Laura for several reasons, none of them as compelling as one would think. First of all, Amanda, her mother, has spoken nonstop about the importance of having "gentlemen callers" and reminisced about having male visitors in her youth. She is embarrassed that Laura has not inherited her popularity as she announces:
It's terrible, dreadful, disgraceful that poor little sister has never received a single gentleman caller!(iii).
Laura feels the need to please her mother and to avoid the wrath of her disappointment. When the opportunity to have Jim as a caller presents itself, she is ecstatic. However, her feelings are not just borne out of a need to please her mother; she has met Jim before, in school. She, a shy wallflower, mentioned she had pleurosis, and Jim thought she said "blue roses." This became her nickname, one which Laura was proud of. Her girlhood crush on Jim never faded, but she never considered herself worthy of him.
In actuality, Jim is simply an office manager at Tom's place of work. His image is enhanced by her brother's recommendation and her own clouded memories of his high school popularity. Finally, her desire to please her mother solidifies her expectations for Jim's calling and ultimately leads to her own disappointment.