If you check the MODIS Vegetation Index User’s Guide (page 9) or the MODIS MOD13Q1 product page you will see MODIS uses a scaling factor of 0.0001 and has a valid range from -2000 to 10000.
In order to get values between -1 and 1, you have to multiply the values by the scaling factor.
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 ...