The coating will not shrink as it cures. It will go on at 16 mils thick and harden at 16 mils. Voids and cracks filled with the wet epoxy might not be filled or sealed when half the product evaporates. Solvent based coatings are also unhealthy to breathe, especially in confined spaces. They also tend to burn easily and skin' over in partially used cans - often resulting in having to dispose of unused open product.
Solvents are one way to thin a epoxy to lower cost.
There are also solvent-free thinners that will do the same thing. All two part epoxies consist of an epoxy base and an epoxy curing agent that reacts with the base resin and produces a hardened, inert product. Special additives are often included to provide special properties and to thicken or thin the epoxy. Sometimes additives are also used to simply lower the cost. Epoxy formulators can select from several different epoxy base and curing agent chemicals.