It's odd to me that colony max size being changed is such a strong discussion point.
First of all, the patchnotes themselves mentioned it's in the configs. You can literally just change the number back to 10. I can understand the desire for it to be inherently supported (as it has ramifications in other systems) and also simply not wanting to remember to edit the config whenever you install the game or what-have-you, but even then it doesn't seem like this should be that big a deal. I would agree, though, that it would be cooler if there were special missions like linking a cryosleeper to a single planet in order to increase it past size 6, but if it's dev time for that versus the actual story.. I think I'd prefer the story? Particularly given that can come later, or from mods, anyway.
On top of that, while we obviously don't have the exacts, from what Alex has said and the patchnotes it seems to me colonies aren't really expected to be reduced in strength compared to the previous top-tier. In fact, it sounds like they may have gone up if you find the right "nanoforge-esque" item. I know for a fact he mentioned one of them "significantly" increases patrol fleet size. Even in the case that their strength does go down, I imagine all the other system changes have shifted what is necessary for defense- though whether that's up or down I've no idea. Once playtesting comes around I imagine he'll tweak the numbers however necessary- though the comments will make that decision more informed, of course.
The way I see it, this is more like renaming the top tier from 10 to 6, simply because the number better fits the numbers in the rest of the game. It's perhaps like if onslaught had 400 LY of fuel range or something equally ridiculous and he went in and realized "oh, that's not really the same magnitude as anything else let's just fix that- clearly should only be 40, just put an extra zero on accident". Not the best example but hopefully the idea is clear.