Here are some photos comparing it with other M.A.C brushes:
From the above, the size of #131 is somewhere in between the #165 and the #129. It's almost a hybrid of the #187 and the #129.
Now, let's compare the #131 with the multi-purposed #129:
#131 is much "flatter" than the #129, while the #129 is much fluffier.
At first sight, I thought "Wow...th bristles are kind of dense.". I was pretty surprised that the fibre optic bristles are quite dense, as I was expecting it to be more loose than this. The bristles have a slight resistance to them when you try to push it (if that makes any sense). So, on the face, you can definitely feel some push on your skin. But fear not, it's nothing harsh. In fact, it is no where near harsh. The bristles are very soft on the skin, and glides on smoothly. *heart*
Okay, back to the dense-ness. So I mentioned in my previous post that I intend to use it with more pigmented blushes (eg. Dollymix). At that time, I was actually thinking of using it like how I would use the #187 or #188. Oh boy...was I wrong. This brush is way too dense to be compared to those two. It is definitely dense enough to pick up quite an amount of pigment. So, you might want to be more careful when dealing with more pigmented blushes with this brush. However, with that aside, this brush is all beauty in one. The narrow bristles allow for precise blush and highlight application, and the fibre optic bristles gives it an airbrushed-look. Very pretty. Just remember to use a light-hand when dealing with more pigmented blushes. =)
I tried this with Cheeky Bronze MSF, using it as a blush. OMG! It is soooo pretty. Seriously, no kidding. The finish was so smooth and looked almost airbrushed. I've also tried this with Dollymix. The duo fibre bristles really gives it a very smooth very blended finish. I was amazed.