Personally, I think it's very rude to tell a pregnant woman she's big, let alone to insist she must be wrong about her dates! Yikes.
'Showing' can vary according to the length of your torso (shorter torsos show earlier), the condition of your abdominal muscles, bloating, hormonally-induced weight-gain, multiple vs single pregnancy... all sorts of things. Might be an idea to check for a multiple pregnancy, but you could just be an early show-er. You might even "shrink" later - my tummy fluctuated a lot with my second baby! It was weird - if I'd done one of those profile-shot-every-day YouTube videos it would have looked most peculiar!
Congratulations, anyway. :) Babies are awesome.