If I'm not mistaken, the reason the logo on xenforo.com works properly is that it's not very tall or wide (120px by 36px). I would imagine if you create a logo on your forum with the same dimensions it would work fine in responsive. However, I suspect your logo is much larger like it is on mine and it doesn't work well in responsive.
I have not been able to figure out how to substitute a smaller logo in responsive and have the header height shrink to the correct size. What I currently do is remove the header altogether when in responsive mode. I can provide you the simple CSS to do that if you like.