Because .NET Core supports self-contained deployment - runtime and framework libraries are deployed alongside the app, so the end use doesn't have to install .NET runtime. Even though this is a great feature if you want to distribute your app to the end users, it has the downside - the size. This is why .NET team developed mechanism called assembly trimming. During the publish process ILLink (the tool that does the trimming) "walk" though the code and identify the assemblies that are used by the code and remove the rest from the result bundle.