Alright, I'll rephrase. There are a zillion differences, but with a couple of minor exceptions, there is very little C++-incompatible C that cannot be straightforwardly expressed as C++-compatible C. I mean, gcc -Wall even turns on -Wc++11-compat and -Wc++14-compat now.
Also, sizeof doesn't behave differently, the arguments that get passed to sizeof are different in a few cases, e.g. sizeof('x'). See Annex C:
It's fine for people to like Cxx, but I do wish we could stop perpetuating the falsehood that C is just a subset of Cxx. Cxx, when it started was a pure superset of C, but that is no longer the case.