[poisondeathray]

It's because the alpha channel is not resized in the same manner. If you view the alpha separately, you will see the problem

not a very smooth zoom either ; I think someone made a smoother zoom function here, maybe riffraff42 or gavino ? Eitherway it's a lot smoother in a NLE or compositing tool
https://forum.doom9.org/showthread.php?t=168266

a=FFVideoSource("video.mp4").converttorgb32

function res(clip clip, clip "LClip", int "width", int "height", 
\           int "centerX", int "centerY") {
    LClip = BicubicResize(LClip, width, height)
    Overlay(clip, LClip, centerX-LClip.Width/2, centerY-LClip.Height/2)
}

function resize(clip clip, clip "LClip", 
\               int "start_frame", int "start_width", int "start_height",
\               int "end_frame", int "end_width", int "end_height", 
\               int "centerX", int "centerY") {
    return Animate(start_frame, end_frame, "res", 
        \       clip, LClip, start_width, start_height, centerX, centerY,
        \       clip, LClip, end_width, end_height, centerX, centerY)
}

b = ImageSource("title.png",pixel_type="RGB32").assumefps(framerate(a))
black=BlankClip(b)
b2=resize(black, b, 
\      0, 5, 5, 
\      300, 1280, 720, 
\      b.Width/2, b.Height/2)

b3=resize(black, b.showalpha, 
\      0, 5, 5, 
\      300, 1280, 720, 
\      b.Width/2, b.Height/2)

overlay(a, b2, mask=b3)


trim(0,500)