I have formed the opinion that population does not matter at all for HGEs. I just plot an efficient route through space with systems filtered to Civilization - Human. This will make sure every system has minor factions/nav beacons, which appears to be all that is necessary for an HGE to spawn. Last night I jumped through 7 systems, and got 7 HGE spawns; 4 of them were in the same system, which was 2.6M population.
HGEs appear to spawn using any of the allegiances/states present in the system, so unless you can find a system with ALL outbreak factions, it's probably not worth looking for them specifically. The only specific HGE I will hunt for is Core Dynamics Composites (Federation), Imperial Shielding (Empire) or if I don't need them, I'll try to stick to Alliance/Independent territory. Boom states are everywhere, they'll happen.
If you need to trade for something specifically, I would run sightseeing missions for BioTech and Exquisite Focus Crystals instead, you'll get 80-100 an hour. Typically I hunt for HGEs till I have all of my CDC and IS needs met, then I switch to sightseeing to get the stuff I'll just trade for.