The Nitro bottles I have come in two small sizes. The bigger size fits snugly in the holster, but I use crumpled typing paper to fill the gaps for the smaller bottle. However, since I wear pants with a watch pocket (like Levi jeans), I am not wearing the holster any more. I just put the small bottle in the Jean watch pocket. It was uncomfortable using the holster under my shirt, because the shirt looked awkward and felt awkward. Interestingly, the big bottle of nitro fits perfectly in the pocket, but I had to put some Velco onto the flap to bring the flap down further on the pocket so it is snug. It works, but as I say, I just put the bottle into my pant's watch pocket now.