Both picture books and magazine stories center around a theme and have takeaway value. They have engaging plots, strong characterization, a specific setting, and lively language.
However, when it comes to magazine stories, there can be more character and setting description and more conversational dialogue. In addition, magazine stories can be understood without illustrations and usually cover one scene.
For picture books, the focus is on one character, his want, and the conflict that results. Writers must consider pace and page turns. Less description is needed because the details are expressed through illustration. Everything is about word choice—word play, rhythm, things like alliteration, and metaphor because picture books are generally 500 words. Dialogue is shorter and snappier.
A short story is usually read just once. A picture book resonates with kids, parents, and teachers and is meant to be read again and again.
I'm all for those who want to pursue writing a book based on a magazine story. Writers can use the same protagonist, setting and conflict, but the story has to be told more succinctly as a picture book. It takes time, patience, and dedication to make that transition. It takes reading books on the craft of writing and studying published picture books to understand what is required of the genre. Because going from story to book takes a lot more than just cutting words.