No worries, it seems that Olivia will be bigger than you :) She will be your little big girl hehe :)

Well, walking is healthy of course but it won´t help you gain some weight. Only gym work out will. I mean I was really skinny, I weighed just some 60 kg when I was like 20 years old already. Now, I look like this: @phortun/workout-update-some-photos-my-current-maxes

Actually, I´m even bigger than that. That post is 2 years old. I have gained even more mass and strength since then. I currently weigh around 90 kg. Only thanks to lifting weight :) Man´s body works differently than woman´s body of course but if you work out regularly, I´m sure that you can gain some muscles as well. I know some girls who achieved that too ;)