I used to think the same thing about Frontier Outposts. It was either waste one of my 5-7 precious colony slots grabbing some useful resources or take the 2 Influence hit to build a outpost, neither of which seemed worth it. Then I realized I could just throw all my developed planets into a sector with a 75% tax and use the freed up colony slots to start another wave of land grabs. Now I barely use Frontier outposts unless I need to grab an area of space that has no habitable planets.
You are tanking your research. Research costs increase with empire size, so having lots of undeveloped planets is baaaaaad. Especially so if they are planets with no research potential whatsoever.
I kind of think they made frontier posts expensive and with a hard limit on purpose to push us towards warfare, though, both for resource systems, pre-ftl civs and better borders.
Oh goddammit that explains so much. No wonder I've been forced to use zerg rushing to win all my wars.
For every population past 10, the modifier for research costs is increased by +2%. So at 60 population your paying 200% of research costs. Keep in mind this is a flat +2% addition to the modifier. It's not 1.02 * (X * 1.02), where X is the base cost.
Say at 10 pop you were generating 15 for all three sciences types. To keep the same research rate as you had at 10, when you have 60 population, you need to generate an additional 15 research in each category.
Assume you divided that 50 population across just three 17 tile planets (Rounding up a bit to 51 pop). That means each planet, when fully developed, needs to generate 15 science. A tile with just a basic science lab, ignoring the tiles natural resource, generates 3 science. That's 5 of the 17 tiles on a planet dedicated to labs to eat the research deficit for the local pop. Not a huge sacrifice for the minerals and energy you get from expanding.
Not only have I ignored the labs tile resource (You should be building labs on sciences tiles anyways) here, but I've also ignored the additional science gained from building research stations in the systems adjacent to the new colony. In addition there are plenty of positive,
percentile modifiers to your science gains mid to late game and you can upgrade you science labs to compensate as well.
What this essentially means is that a large nation and small nation can
relatively compete tech wise. IE: Small nation that spends 30% of its income on science == Large nation that spends 30% of its income on science.
In short, you're fine expanding if you just make sure to not neglect your science income.