I was slightly underweight with the girls, but very tall. I didn't start showing til 5.5 months. (Just looked like I was getting fat at 6 months, lol)
I started showing a week or 2 earlier with DS, but I was a bit bigger with him. And looked pregnant. Not like I was just putting on weight around my belly, lol. Carried completely different. Alot lower & out more. The girls were high & hiding, lol.
I think its all about your body shape more than anything, & your muscle tone between babies helps. I was alot fitter when pregnant with the girls. I was pretty unfit with DS had put on about 10 kgs since DD2.