There was a woman in my birthing class who had the same sort of build as you, she was further along than any of us and her bump was the smallest! The biggest bump belonged to a much shorter woman who was also rather slight.
I'm kind of the exact opposite of you - fairly short (5'5) and was overweight before I became pregnant. I started showing VERY early, though I think a lot of that was that I retained fluid like nothing else. In all fairness though, I don't know if I just looked pregnant to myself because I knew i was pregnant. I had someone ask me when I was due when I was maybe 10 weeks at the most, though by that stage I was rubbing my tummy and just generally acting pregnant lol. But then I had people tell me when I was 26 weeks that I didn't even look pregnant!

I'm waffling now. My point is, "showing" is very subjective hehe.