Yes that would probably be because of the colonization range distance you set when creating your new game.
On the previous game I set the maximum range to 1 (sector.)
But the problem I had was luck became too much of a factor, since empires with more planets to colonize nearby
could spread faster, since it expanded their colonization range, but empires with few or no planets to colonize
nearby had to wait it out until they got the appropriate tech.
So my empire had 1-2 planets, where my neighbor had 8-10.
In my next game I'm trying 1.5 distance to see how that pans out, but it also depends on number of sectors
I remember when this feature wasn't even in the game, and empires would be colonizing planets everywhere, far
away from the rest of their empire; so I very much like that the feature is available.
So it would probably be up to your personal preference what you'd like to set it to, if you're having difficulty
colonizing you could try turning it up, however you can only set it when starting a new game.