Having a larger bump sooner during your 2nd or 3rd pregnancy is quite normal. During your first pregnancy, the muscles in your womb (uterus) and tummy will have stretched. You may also have a little more tummy fat this time. This is especially likely if you became pregnant before losing any excess weight you gained in your last pregnancy. 

This is not a cause for alarm because there is really no conventional way to measure the size of your belly.

The position of the baby in the womb can also determine the pregnancy bump size. When you conceive the second time, your abdominal muscles are not as attached to your spine so the muscles are not able to support the baby as before. As a result, the baby drops lower in the abdomen, making it appearing it to look larger.

The way you carry your body – yYour posture greatly affects how your tummy protrudes. When you stand, sit or move in a drooping position , the stomach muscled becomes more relaxed, increasing the possibility of athe bigger pregnancy bump.

Excessive amniotic fluid can also affect the size of your belly bump, while for most this is not an alarming concern but do consult your OB to be sure.

SOURCES:

https://www.beingtheparent.com/is-it-normal-to-have-a-bigger-bump-during-second-pregnancy/