Leather holsters out of the box tend to be a pretty tight fit, this is so the holster will mold itself to the gun over time, sometimes called "training" the holster. this is just getting the leather to stretch just the right amount so your gun slides in easily but still has some retention so the gun doesn't just fall out if you stumble or something.
There are a myriad of ways to train your leather holster; I wrap some wax paper around my gun and then insert into the holster and leave it there for a couple days. Some people use a plastic bag (like a ziploc), some draw and insert the gun several dozen times while wrapped, and so on. I think you can do a search like "leather holster fit," or something similar and find several threads on the subject.
Or just wait a while. Everyone has there favorite way of doing this.
