The problem is that you’re basing your purchase on how your jeans are fitting when you first try them on. Denim is made of cotton, a natural fiber. Denim will naturally stretch with wear and shrink with wash, but pure denim will usually wind up stretching over the long-term. How much will depend on how the denim was treated and how you wash it.
If you’re looking at pants that are 85% or more cotton, you should consider sizing down a half to whole size. When you try them on in store, they should be snug and tight, but not uncomfortable. Note where it is the tightest and ask the SA if they have any knowledge of how much the jeans will stretch. When washing jeans, I prefer cold washes and hang dry, to minimize the heat and stress placed on the fabric (i.e. the effects that alter and, ultimately, shorten the lifespan of the fibers).