It can be diffrent for every pregnancy, but typically you are going to show sooner during your second pregnancy. Baby is going to grow along with your uterus and everything it contains, so even with losing weight you are eventually going to show because you have a baby growing inside of you. I was showing by 10 weeks so 17 is not bad at all.
It's also your second pregnancy. With my son I didn't show til 6-7 months. And I had lost a lot of weight with him. And now I'm 25 weeks with my daughter and I've been showing since about 16 weeks. And I lost about 20lbs in the first trimester with her.
Because the baby is still going to gain weight and get larger, causing you to have a belly.