But having less POP means that the techs are cheaper, meaning you don't need as many new laboratories as a "wide" nation to research them, so increasing buildcost would not affect everyone equally harsh.
Intelligent, natural physicists/sociologists/engineers traits and the materialist ethos all increase tile output of "science resources", meaning in that timespan of the game where you can't build laboratories they would have a significantly higher (specifically +20%/+20%/+35% in any order) output of "science" than everyone else for much longer, which would cause snowballing with those traits to be much easier.
Especially in the mid-late game if you choose to go for a "tall" empire instead of a "wide", since everyone else gets an increasing "research speed" penalty in addition to not being able to build/upgrade as many research stations and laboratories as they normally could to compensate.
It's not that the immediate impact is ridiculously large, it's the fact that the potential snowballing is incredibly OP.
I don't think that is how it will turn out... here is why
Say you start with 10 pop that deliver 10 research in each category.
After X amount of years you have five worlds and research stations that provide an additional 10 in each category and increased population to 20 for an additional 10 research.
This is a total of 30 research, let's say we give a negative modifier for population of 5% per 10 pop so increase tech cost from 100 to 110 points
If you have a higher pop growth and instead now have 50 pops you get 60 research at a 25% increased tech cost so that give us a 100 tech at 125p
Being intelligent just add 10% extra research in both cases so in the first case you get +3 points and in the last you get +6 points for a total of 33 against 66 points.
These are just numbers to show how the difference will be... the offset for population will never be as strong as the additional research you get. You have been able to see some of these numbers in the stream and the penalty on research for population growth are not that severe that it completely stifle research in any way.
Now also add the fact that all research labs, stations cost twice as much to build in time and minerals.
As far as I know traits such as Intelligent are a simply bonus to your overall tech output so it will effect either case equally much.
If you also like to add some cost to tech will just be a balance issue... trait can also be balanced but I see no reason they would impact anything in particular either way.
The change in pace here will mean that the difference between a tall and wide empire will be less in the middle game since it will take longer for them in relative terms to reach a peak in population numbers and will be able to progress further through the tech tree before a wide empire start to generate more science in regards to population.