How to export LIGHT video with transparency?

Hello, 
I've created some small video (5s duration) that I would like to export in order to use them again inside my other videos. The problem is that,  even if so short, the weight is around 250MB. (at 1080p 60frame).

Having 6 of them, is 1.5GB , and it is absolutely not convenient for my workflow. 

As export settings, I use appleprores 4444 8bit+alpha 

I also tried to use webm, but when I've included the exported videos on my new timeline, no transparency was applied.

Is there some other codec, or option, that I can use in order to obtain smaller files?