"If it's too heavy, that probably won't work." Mika wasn't really all that physically strong, and she'd prefer a weapon she could wield without resorting to psychic assistance. While she was capable of using something large or heavy relatively easily due to her powers, there was a limit on how much psychic stuff you could do at the same time. This was generally limited by how many things you could focus on, and having to keep psychic focus on her weapon at all times would reduce Mika's options in general.