I think it could stand a bit of improving so it's less of a case of you being completely screwed at the smaller end, while being untouchable at the larger end. Make energy a bit less scarce for people with few planets, and more scarce for those with like 20.
I mean, look at it this way; At few planets you can concentrate all of your defenses in a few places, and will be suffering smaller attacks, so you are less likely to lose out on power generation. However, because you have less power, you just can't power all of your defenses at all. You have fewer resources as well, so matter converters will hurt. You also can't get decent defenses because of little knowledge. Top this with the fact that if you lose even a single planet, that could well be game right there. There's just no wiggle room.
At many planets, your defenses are more spread out so you have to rely a lot on chokepoints. However, with the use of chokepoints, you fall back into the few-planet advantage of all of your defenses in one place. However, you have more power to power it, more knowledge to research it, more resources for matter converters, and a bit more of an energy buffer to live with so if you do lose another planet then it won't necessarily ruin your defenses. There's the problem of suffering larger attacks, but you needn't worry about that when you already have more advanced defenses as a result of that.
There's more to it than that, as I'm sure someone else will helpfully describe, but that just seems a bit off to me. Don't get me wrong, it's better than being what used to be a completely unstoppable energy goliath, but the curve's weird.