This is an opportunity to outline positive benefits you hope to gain by transitioning into a new role. However, one of the biggest mistakes you can make when answering this question is focusing on negative aspects of your current employer, rather than discussing positive aspects of the new company.
An HR professional will mark it as a red flag if you talk poorly about your current employer or company. It illustrates someone who isn't very professional, has a negative attitude, and could bring toxic energy into their new work environment.
To avoid those traps, say something like this -- "I appreciate everything my current employer has done to help me grow, and I believe working for a small start-up over the past year has helped me develop leadership skills earlier than I could've at a larger corporation. However, I am now interested in transferring the skills I've acquired here to a larger organization, where I believe I'll find more growth opportunities in the future."