Is C++ inherently slower or bigger than C on a microcontroller?

No, not the language itself. C++ is designed around the zero-overhead principle: you don't pay for features you don't use, and the ones you do use compile as efficiently as equivalent hand-written C. A non-virtual class method is just a function with an implicit this pointer; led.toggle() generates the same instructions as led_toggle(&led). Bloat comes from specific features, exceptions add unwind tables and code size, RTTI adds type metadata, <iostream> pulls in an enormous amount, and dynamic containers bring in the allocator. The embedded discipline is to use the cheap abstractions (classes, RAII, templates, references, constexpr) and disable or avoid the costly ones (-fno-exceptions -fno-rtti, no heap). Done that way, C++ is the same size and speed as C with better structure.