Why do you say that? Certain things in a language may increase binary size or something else but at the end of the day these languages all go through most likely the same compiler and should produce a roughly equivalent binary.
For example, GCC's C++ front end will produce a different parse tree for the same C code as the C front end, resulting in a slightly different collection of basic blocks and data flow analysis (obviously, for very trivial examples, it will be identical). The result after all the gimplification and and different middle-end passes and rtl transformations you can end up with a surprisingly different set of generated instructions for the same code fed to gcc and g++. Equivalent, but different.
The fact is, the front ends of cc1 and cc1plus will interpret the exact same C code differently to the extent that code containing undefined behaviour will have remarkably different results in the generated instructions. The differences between the cc1 and the cc1plus compilers are not limited to what the compiler driver passes to the linker, they have completely different front ends and only share the middle and back ends.
Sure, the cc1 and cc1plus binaries come with a middle and backend linked in, but what differentiates them is the front end. Not so much for gcc and g++ where both `gcc file.cpp` and `g++ file.cpp` will invoke cc1plus. Not to mention that with LTO cc1 and cc1plus do almost exclusively front end work while everything else happens in lto1 (for both C and C++).
That's not really true. A single compiler may support many languages but that does not mean code written in those languages ends up as the same binary.