I have run the big fat Pro Grips for almost four years, and don't get blisters, sore hands, numbness or anything. I also run ProTaper bars, and have for years. A week ago, I ran the 108 mile National Hare & Hound and never got arm pump or fatigue (although I did manage to sink the 500 in the big hole on the first water crossing. Turning a 500 upside down...twice is a lot of work). Also burned a lot of gas in the desert last Saturday and Sunday with no problem.
It might be that you are hanging on too tight or have the wrong riding position, as both of these will impact arm fatigue. The first six months on a 500 was tough for me, as I was nervous about the power, so would not ride over the front fender. Work your way up to riding over the bars, and your arms will be at almost a right angle to the ground, so very little arm effort is required.
As a note, Ben, my 14 year old son, has had problems with blisters riding/racing his KX125, and always wanted to run the thin Scott or Renthal grips. For the 108 mile National a week ago, I talked him into running a set of the fat Pro Grips. Not only did he not get blisters, he finished 20th overall! Not bad for a 14 year old on a three year old 125. (Before the race, he was concerned about the "fast guys", so I remined him at the end of the race that the only guys who beat him had names like Brown, Pearson, and Abbott, so he's probably one of the fast guys the rest of us all worry about.) I think he is now sold on the fat grips.
Good Luck in your quest for comfort...
Rick