Use Haiyuan Ai to investigate first. Actually, it is understandable for Conan whether he chooses Haiyuan Ai or Mao Lilan. After all, both of them are very suitable for him, but he and Haiyuan loved to stay for so long and chose Mao Lilan. I think it may be because in Conan's mind, he and Haiyuan Ai are just cooperating in the investigation. Because both of them took smaller pills, they are now acting like primary school students. Although they can't change back to the way they used to be, their hearts will definitely remain the same, and their memories will not change, but their bodies will become smaller.
Second, Mao Lilan was a childhood friend. For Mao Lilan, Conan must be very distressed, watching her wait for Kudou Shinichi for such a long time. Although he is with Conan, he always thinks about Mao Lilan. He and Mao Lilan have known each other since childhood, which can be described as childhood friends. When I was a child, Kudou Shinichi's personality was actually very proud, while Mao Lilan loved to cry, which is why they met. And the two of them can be said to have watched each other grow up and been together since childhood.
Third, Conan, who can't be Joseph because of necessity, must really want to go back to Mao Lilan, but the two of them are just because of Conan, and he is getting smaller and smaller now, and he hasn't figured out what the real purpose of the people behind the scenes is, for fear of appearing rashly, which may bring fatal disaster to Xiaolan. So, for Conan, after a lot of ideological struggle, he decided not to tell Mao Lilan. They must be together, not together.