First, your file sizes are as expected. When you go from 1000m per pixel, to 30m per pixel, you get 1111-times increase in size, which is close to what you see from 2 MB to 3.4 GB, once you also consider compression etc. The calculation is:
(1000 m/ 30m)^2 = 1111.1
It is squared due to the raster being a 2d array.
The whole point of STARFM, or ESTARTFM (a ...