I think it'll be more like 1..3 MB for something like the .NET mscorlib, but yes, the runtime overhead cost is definitely there. The more "managed" the language is the worse this will be. That's why it makes a lot of sense to use 'bare metal' languages which have a very small, or even completely optional runtime for WebAssembly, and use an 'embedded platform' programming style, that way you can get useful WebAssembly modules down to a few dozen kilobytes.