What am I doing wrong???

[Imp]

Firstly I hope you added a zero to that number & meant 6,500.
As your force is now elite it is worth quite a bit more, from memory a unit with 100% exp is worth 50% more than one at exp 70. So that would be a 10,000 point force & rightly so experience counts for a lot once it climbs over 90 or so.
The AI uses fairly realistic forces & as you rightly say the cost diffrence between units means it will get more soon reaching the unit cap of 500 when you will have an advantage because it cant spend all its points.
Its equipment is generally cheaper than yours if bought high end stuff & thats without the experience boost which the AI does not get so the AIs force growing is fair as it attempts to balance the game otherwise its probably lost before the battle starts.
So you are doing nothing wrong & in my view the cost for high exp units (90+) should be far higher as they can stand up to standard units with ease so you can win the battles & if you lose the odd one well thats war.
The AI will not change as it uses sensible forces if a bit tank heavy in general even on standard setting but I must admit experience gain for your units is tailored to short campaigns in my view & does not work for long campaigns. They need to gain exp much slower once get over 80 as after about a dozen battles they become supermen & a platoon of infantry can kill a enemy company with ease. Due to the fact they will probably have a third more shots than regulars & can use them all as shrug off incoming fire where as the regulars will often give up after taking 2 shots so you actually have more firepower. Same problem applies to units with guns the extra shots it gains are worth a lot in WW2 as get far more 3rd shot accuracy fires making them lethal, just keep an eye on the ammo.
The only way I have found to make long campaigns playable with sensible units is after about the 8th battle sell some off & replace with standard units & keep working through your force replacing some each game. Of course the AI might save you the effort & do it for you.
Andy & Don beat me to post so probably repeat points they noted