One thing that isn't mentioned in this question about Ultimate Meters is how environmental kills affect the Ultimate Meter. Specifically, does the HP of the hero being killed affect how much charge is gained? Is it better to boop a Roadhog off the map than a Tracer (as far as your ult meter is concerned)?
It would appear as though the amount of ultimate charge is granted based on the current health of the target when the kill is registered by the death-zone at the bottom of the pit.
Test case 1: Winston (500HP) Environmental
Test case 2: Winston (~240HP) Damage+Environmental
Test case 3: Winston (500HP) Full Damage
Test case 4: Winston Primal Rage (1000HP) Environmental
Test case 5: Tracer (150HP) Environmental
Test case 6: Tracer (120HP) Punch+Suicide
We can see from these clips that, negating the over time ultimate gain, the amount of charge gained is consistent with the total damage dealt, not including any measure of overkill.
For reference: For every 15 points of damage dealt by or as a result of Pharah's actions, 1% ultimate charge is granted. This means 33.3% charge for Winston, 10% for Tracer (consistent with what we see above).