“I’ll Leave It to You” Means You Can Decide

“I’ll leave it to you” does not mean leaving an object in someone’s possession. It means “you can decide” or “I’m giving you responsibility for handling this”. The preposition to marks the transfer of the choice or responsibility.

For example: “Which layout should we use?” “I’ll leave it to you—you know the audience better.” The reply conveys trust: you understand the readers better, so you should choose. But if the other person needs your opinion, it can also sound as though you are handing the difficult decision back. If you simply have no preference, “Either is fine with me” states that more directly.

https://dictionary.cambridge.org/dictionary/english/leave-to