Fat bikes are made for low psi, used to run low psi on my motocross cross bike in the mud, get better grip. Street trials is quite high psi due to the need for low rolling resistance and sharp edges, if you ride rocks and stuff though natural trials, you may want to drop the pressure a bit so the tyre moulds to the surface a bit better, it's all personal pref really.
If your arms are getting pumped from gripping, either fat bike or trials, you are holding on way to hard, try and relax and loosen up slightly, will improve bike control.