Limiting PLANETS or SIZE?
Limiting Planets. If you want a larger map but lower planet numbers, play with tight clusters and habitable worlds set to the lowest level. I did this once on immense, and there were maybe 30 stars across the whole map.
More planets doesn't do much for me, because I hate having too many planets.
Good thing you aren't the only one playing the game then, huh? They CAN and SHOULD design things so everyone can enjoy it. By altering the settings, you can get your 15 decent worlds, but to me, as is right now, Immense feels gimped.
IMO having more than 15 'decent' planets begins to ruin the game.
And I feel that until my 'empire' hits 100 worlds, it doesn't justify the title. Until then I'm not building an empire, maybe I'm building a sub-division. Epic fleet battles is why I play. (and yes, I understand not everyone feels this way, but in-game settings exist to make the game more like what you want)
Besides, if you want to play in a galaxy with 15 decent plants...(9 of them being other races homeworlds, leaving only 6 free to colonize) an immense map might be a bit TOO big for that. (it might even be too big if you meant 15 decent planets per side (which is not what you said) which would be about 165 planets)