Each animation frame has the black background included in it? Create the stars on transparent canvas & add a static black image as a separate object. Removing the black from each frame will significantly reduce the file-size of each frame, by quite a far bit I should imagine.
Would you be willing to show or send me the animation, so I can see exactly what it looks like? If you do I might be able to give you a quick Lua script to emulate the same effect, with just one animation frame.
* edit: if you just want it to move down then you only need one line of code & a single animation (I recommend animation over pure object with static image as animations are much easier to move with absolute positioning). Technically you can move objects or animations with action parts too, but they don't have easing options.
local anim = ActiveAnimations["stars"]
anim:to(3000, {AnimationCurrentPosition = {x = anim.AnimationCurrentPosition.x, y = 2000}}, easeQuintOut) -- x stays the same, y moves to 2000px over 3000ms, with slow ease out.