Better they'd restrict the number of capital building thingies that can be built per consellation or per region or something. Increase building times etc.
I mean, I dont think the Allies called the German during WW II saying: "Say, those Tigers are too tough, cant you put thinner armour and less powerful guns on em, so we have a fair fight?" If the more powerful capitals are as powerful as they are, they should be hard to build, be costly etc. It seems to me that this isnt the case atm, because there are so many of the things around. It's not like the Germans built 1000 Tigers each month.
Production of the Tiger I began in August 1942, and 1,355 were built by August 1944 when production ceased. Production started at a rate of 25 per month and peaked in April 1944 at 104 per month. Strength peaked at 671 on 1 July 1944. Generally speaking, it took about twice as long to build a Tiger I as another German tank of the period. When the improved Tiger II began production in January 1944, the Tiger I was soon phased out.
Story behind it could be that there is only so big a workforce in a constellation to work in a capital building thingy. This way you can even connect things like PI and Dust to this mechanism, where more developed planets in a constellation can support more building thingies, or have them work more effectively.
A ship being very powerful compared to other ships is not a bad thing in itself. It's when there are dozens of em in one fight when it might become a problem.