In episode 177, "Reunion with the Black Organization", Conan asked Haibara to drink the liquor which made her restore to her original size, but why did not he tell her that the first time he met her or at least after he trusted her? This is a very important information which cannot be delayed.
Remember that once your body develops an immunity against baiju thus you can't grow twice. Conan was probably thinking that it might be useful in the fell clutch of circumstance.
Plot-wise, if Haibara already drank baiju and so, if Haibara wasn't in adult form, Conan and Haibara will be unprotected and might die. Since they have this plot immunity, then that won't happen.