You can change how much each lvl gives you Fleet supply and how much each lvl takes from your income.

But you can not change it. Its core to the game. Its a design feature that in place to help bring an end to game, prevent spaming units non stop on a small empire, and promote more not throwing away large fleets.

There been many of discussion on this topic but its not gonna change, the most you could do is tweak the cost per lvl, for a mod.

I wouldn't mind the ability to temporarily increase cap, at an increased tax rate (greater than permanently upgrading your cap), for dealing with early-game invasions. I hate the moment when you realise you gotta take a permanent economy hit, which you probably wouldn't need for another 30 minutes if this tiny assault force hadn't turned up at your capital.