Use bar tape for your mtb handlebar, instead of rubber grips.
I have small hands and find that many rubber grips for mtb handlebars are too thick - especially when I'm wearing gloves. I also find them to be very slippery in rainy or muggy (sweaty) conditions. Therefore, I use regular bar tape (cork or micro-fiber) instead. I wrap from the inside of the bar (nearest the stem) to the outside of the bar. The resulting smaller diameter suits my small hands better, and I find the grip much better, with or without gloves, when it's raining or I'm very sweaty. To each his own, but give bar-tape a try and judge for yourself.