AT THE SAME COST, A civ that controls 1/3rd of the galaxy would be able to get the needed resources for MUCH MUCH MUCH faster than a civ that controls only 1/8th of the galaxy.
You're not understanding the concept here.
Let's say we have two runners, A and B, running in a race. A runs at 10m/s. B runs at 5m/s.
After 1 second, A will be 5 meters in front of B.
Now, let's put a barrier in the way. 10 meters from the start of the race, we will force everyone to slow down by 50%.
A hits that barrier after 1 second, thus slowing down to 5m/s. So, after 2 seconds, A will be... still only 5 meters in front of B.
A lost an entire second's worth of progress.
Now, when B hits the barrier, he too slows down. After 3 seconds, A will be only 7.5 meters ahead; without the barrier, he could have been 15 meters ahead.
That's what I'm talking about. A is still ahead (this is important; you can't stop A from progressing faster relative to B, or else there's no incentive to improve), but he's not as far ahead as he could have been. Diminishing returns.
It's only negative for civ's that are ahead, it is positive for civ's that are behind.
Arbitrary rubber-band rules like this are not appreciated by any player, even one who is losing.
Never try to tackle a problem like this directly. Game design must be subtle; if the player can easily tell how a rule is supposed to affect the game, it's probably a bad rule.