Ocean (frex) + Accident = Radiated (smaller)
Radiated + Atmos Terraform = Minimal (larger)
This is the relevant element.
Here's how Atmosphering Terraforming works.
* ANY spending (even 1 BC) to Atmospheric Terraforming transforms a hostile-class planet to a minimal environment (and increases the base planet size), but it will still be rated Hostile.
* ALL of the spending must be completed to get rid of the Hostile environment (pop growth halved).
* Barren, Tundra and Dead planets have base pop increased by +10 when they are improved to Minimal.
* Inferno and Toxic planets have base pop increased by +20 when they are improved to Minimal.
* Radiated planets have base pop increased by +30 when they are improved to Minimal.
Thus, when the Accident hits, if (and only if) the planet's base pop shrinks by less than 30 units will there be a net gain upon arrival of Atmospheric terraforming and removal of the Radiated condition. It all depends on the initial base pop. The +30 for terraforming from Radiated to Minimal is universal, ignoring base pop. (A radiated planet with base pop 10 goes to base 40 upon atmospheric terraforming. A 60 goes to 90).
Soil enhancements increase pop relative to BASE pop size. Soil Enrichment turns a planet Fertile. Advanced Soil turns a planet Gaia. Fertile planets increase base pop by +25% and Gaia by +50% (not cumulative for both. Gaia is an upgrade to Fertile, and Fertile can be skipped).
So turning a size 20 to "fertile" will increase it to 25. Turning a size 60 "fertile" will increase it to 75.
Turning a size 20 that has +40 terraforming to fertile will NOT increase it to 75, but to 65. All +n terraforming is added to the base, and not calculated in soil enrichment. Only base pop is factored.
So it kind of goes like this:
BASE POP = N
SOIL ENRICHMENT = S
ADDITIONAL TERRAFORMING = A
Max pop = (N * S) + A
Atmospheric terraforming increases the value of N, according to the chart I listed above. (Radiated gets the largest boost).
The Accident event shrinks the value of N.
Sometimes it increases planet size in the long run, sometimes it doesn't.
The smallest planet in the game is a planet that STARTS as a Minimal, with size 10. It's base pop will never grow, and even with Gaia status it only picks up 5 points. Max value of A is 120, though, with Complete Terraforming tech at the top of the Planetology tech ladder. So even the smallest planets in the game can reach total size of 135. That would be 10 base, 5 from Gaia bonus, and 120 from pluses. The largest possible planet is base 120, plus 60 from Gaia bonus, plus another 120 from pluses, for a total of 300.
Radiated planets can be as large as 60, I believe. Or maybe they can get to 70. With the +30 bonus they get from moving to minimal, that could be base pop of 90, maybe 100. But even a size 10 toxic (a common occurrence) will grow to base size 30, to 45 with Gaia, and up to 165 max. Thus, yes, you want to grab every bit of habitable rock you can get your grubby mitts on, because if the game goes deep into the tech tree, there's no such thing as a junk planet. Even modest terraforming available early in the game can make any rock worth your while.