I think it depends more on the length of your torso - how much you expand outwards depends on how much your insides can move upwards.

Anecdotally, one of my mum's group mums is skinny as a rail, she's currently twenty-ish weeks with her second, and has just barely started to show. She has a very long torso.

I am about seven weeks behind her, have a similar build and height but a shorter torso, and have been showing since about seven weeks.

Although I don't know if her uterus is retrograde - mine is though.

I hope you get your belly soon gorgeous - but just remember, before it gets too big, say hi to your knees and toes. It gets to a point where you forget what they look like.