Don’t understand baby size comparisons

Je

All of these apps I’m using tell me how big my baby supposedly is. I’m 19 weeks, and this week it’s the size of a mango or a view finder. But I don’t see how it’s possible. I’ve barely gained any weight maybe 8 or 9lbs). Do these sizes include amniotic sac or is it actual baby size? I’m worried something is wrong since I can’t visualize it. I don’t even really get pains from my body growing. Only constant boob pain.