I am almost ready to order xt660z and the last concern is for me is if that bike is top heavy? I mean that big gas tank is not light for sure, isn't this a problem off road?

I've been riding mine off road today with a full tank and it's not a problem. I don't find the bike especially top heavy. I've ridden plenty of others than have been, but the XT doesn't especially strike me as a top heavy bike.

thanks that reassuring because I don't have an option for demo ride
kinda have to rely on opinions of others before I buy it. That 23 liters
of petrol will be like 16 kg weight on top that's why I'm concerned.

These things are all relative of course. My XT is much easier to handle, feels lighter and is more fun to ride on my trails than my BMW GS1200 Adventure was, but the XT with a full tank will no doubt feel big, heavy and unwieldy compared to a mid size enduro bike. It's all a compromise somewhere and just depends what your priorities are.

Certainly, compared to the BMW I've not noticed the effects of a full tank of fuel in quite the same way on the XT. But with a big tank you don't have to fill it right up all the time if you are concerned about it.