First of all, orks are an infantry race. Looted tanks are wannabe Leman Russ and cost same cap (and their innaccurracy makes them useless in conjunction with ork infantry), Ork kans aren't as good as dreads and cost same cap, and the squiggoth, while awesome, is a unique unit.
Now, comparing the numbers of basic orks I can field with the number of basic space marines, I see that I will hit the space marine cap and the ork cap with the same number of shootas vs. space marines, i.e. 8 marines and a sergeant vs. 8 shootas and a nob. 8 shootas and a nob don't match up to 8 marines and a sergeant, unless you're committed to melee (which can't happen for every ork vs. every marine for the practical problems with mass melee).
Additionally, ork heroes and leaders pull away from the cap, shortening it even further.
So, basicly stated, the space marines can field just as many infantry as orks using permanent upgrades rather than fragile buildings, and then a few extra leaders, whilst having somewhat more effective infantry and considerably more effective vehicles.
A larger ork cap would be automaticly limited by the need to build and protect even more Waaagh! banners to make it possible. But now you would be able to field a 16 unit slugga/shoota boy squad for every 9 marine tact squad a space marine could field.
Nobs in a squad (not leaders) might should be upped to 3 cap, I dunno.
A further balancing act would be to limit heavy weapons available to an ork squad according to the number of orks in a squad, so said orks can't lay out the theoretical 105 rokkits or big shootas that would be possible, or even the 75 that are possible right now.