Personally I don't care if the whole "More Battles = Bigger Orks" thing is implemented, but my suggestions if it were:

Have certain classes have a noticeable limit to how big they can grow.
Have the hitbox size grow as well.
To compensate for the increased hitbox also have their natural resistance and strength increase, making them slightly tougher and able to carry a bit more weight.

Orks will have something like a "Battle Bar/Gauge", which starts filling up as they fight, with small increases against NPCs and large increases against other players.

When it fills up they start to grow, and the bar drains, requiring them to fill it up again to grow more.

Dying should set you back by one growth per death.

Ah, did not realize there was a thread about this already.

Lol... What about the one guy whos amazing and kills like 100 people a in a row. Hes gonna be able to step on fortresses XD

1 death lose one growth is a bit difficult considering that dying is going to be very common

And if you die more then you kill you become a gretchin