Mikio and Mariko are childhood friends who no longer get along as well as they used to; Mikio makes fun of Mariko about her bra size, while Mariko teases Mikio about his short stature. One day Mikio awakens to find that his body has shrunk in size and he is now a miniature version of himself! He finally manages to make his way to Mariko's house after nearly being eaten alive by her cat, and explains the situation to Mariko - after which she decides that she will keep him as her pet! Now that Mikio must depend on Mariko, he begins to realize that things were not as they appeared to be...
In both manga there is a similar hight disproportion between the male and female lead and that's really all you need to know.