If you add more virtual functions, the class size does not grow though.
That can be a little misleading. The size of objects (class instances) does not grow, but the vtable size changes, and maybe even its layout changes. That's important to know, because it's an ABI change and the not infrequent cause of subtle and inexplicable runtime failures.
This sort of thing shows up as a problem if, for example, you add a virtual function to a header file and rebuild a program, but do not rebuild a DLL that also uses the header file. Bam!