The weight adds up deceptively quickly.
Speaking as a fairly large individual, 18" and ~2.5 lbs. is the practical upper limit for most khuks in general and the AK in particular; a few inches longer is fine, but the weight is pretty much nonnegotiable. Beyond this and the khukuri will become tiring out of proportion with its effectiveness. If the job is going to require more than a few blows, I often find that a smaller and lighter khukuri will finish things more quickly than a larger one as I don't have to take breaks.
One trap that I've fallen into (and I suspect others have as well) is to pick something up, swing it around a bit, and say, "This is fine. I can handle this," only to find out thirty minutes later that chopping for half an hour brings different variables into the equation than a few test swings at a log do.
I myself have a few "superheavies," including a 25" CAK by Khadka. It is a beautiful piece, excellently balanced, and even a bit light for its size (and feels yet lighter because of that balance), but I would not want to spend a day in the field with it. JMO.