Yes, they all have different research determining what the fleet supply/capital ship limit is. Even though in game this is only used to show different icons, it is very easy to just change the numbers (fleet supply given/cost/#of labs etc) as well.

RESEARCHSUBJECT_MAXSHIPSLOTS[RACE]0 - 7: Determines how many supply slots per research level and taxation.

RESEARCHSUBJECT_MAXCAPITALSHIPS[RACE]0 - 7: Determines how many capitalships one is able to field per level.

Changing these changes how great your fleet supply is and the number of capitals capable.  Weak race 'A' would not necessarily have fleet supply different from strong race 'B'.  Race 'A' ships would have less fleet supply than race 'B' to compensate.

If you just wanna go nuts, have each level give 10000 fleet supply and minimal taxation and each capital level give 100.  Good luck with having 700 capitals and 8000 ships fighting another 700 caps and 8000 ships -it might blow the 2GB mem limit or melt your computer.  lol.