Frankly, there is no valid reason for starting a new program in C in this day and age.
A few years ago I would have agreed with you, but these days I'm not so sure. Even if you would have stayed away from templates, virtuals, exceptions, RTTI and other features that obviously impact size/speed, I can think of several possible reasons to stay in C:
1. C is the lingua franca of languages; if you write a module in C, pretty much everything else can call it without too much effort. A C++ API, on the other hand, can't easily be called from anything except C++ (and preferably C++ built with the same compiler and options). Yes, you can hide your C++ implementation behind a C interface, but that's not free.
2. C++ tooling is improving (LLVM's Clang in particular looks very promising) but basic text-processing tools work a lot better for a language without overloads etc. Think grep, ctags and the like.
3. C++ is a huge language, and people tend to settle into their own subsets and idioms to make it manageable. For a solo project that's fine. For a big group project, especially one without a recognized benevolent dictator, it's a recipe for pain.