IES file is the photometric presentation of the luminous intensity ( cd or lm/sr).
If you open the ISE file with the notepad, you don't see any info about the size of the real light source.
You can create 5x5 CoronaLight plane or 200x200 and you will have the same amount of light, if you will use lumen as units.
For the reflection,. Corona will use the size of the CoronaLight. This is normal.
But, if you will check the option "Keep sharp pattern" Corona will convert the CoronaLight (Rectangle or Disk) as a Point-light and you will not see any reflection on the objects.