Increase size of the fruit as Y value goes down(up the screen), seems easy enough. Then when it hits a certain Y value, stop changing the size. Because you've used a bullet, you could set the Y starting location as a variable when it spawns, then as the Y value reduces every tick, increase the width/height by 1 every tick, or slower than this if you want. Then when the fruit is at a certain Y value, say the middle of the screen, stop increasing its size by toggling a boolean to lock it out.