I had a mini nikki and now I have a regular nikki. The reason I sold my mini was because of the leather itself. It was stiffer and didin't slouch well, which made it uncomfortable to carry. I'm not sure if all minis are that way or just he leather on the one I had. It was a nice size though. Probably holds a little bit less than a MAM, but I never fill my MAMs so it worked out for me size wise.
The nikki I have is really smooshy, so it doesn't always look as big as it is. I do like to put a purse organizer in it though or I would never find anything. If the Nikki came in the MN in the leather I have, I probably would've tried it, but I don't absolutely love huge bags. I have a mini Rikki, which is the same size as the mini nikki and it has more slouchy leather and is great. So I think it really depends on the leather.
Full size Nikki pic. I'm 5'7
Mini Nikki - see how this leather looks stiffer.
Mini Rikki is the same size, but this one has much better leather. I think the trick is to figure out which size you love more and then seach for a good leather.