I want a chubby healthy baby !

Balha
Hey ladies I'm just wondering & looking for some thoughts .. I'm naturally lean and very small , 5'4 and 116lbs .. when I get pregnant I add on about 20 lbs and have 3 kids and number 4 is on the way .. my baby bump always always is huge and makes me look funny , and my babies are always small but healthy lol .. 
Do you guys know if it's me , not eating well enough calories , that's causing my kids to be little ?should I eat more than usual ? ? Is working out a reason for up-bringing tiny lean babies ? 
Is there such a thing that a pregnant woman does to have " chubby " healthy kids ? Haha just wondering ..