Having gone through it, while it's true you need more calories after the baby is born (I breastfed and pumped exclusively and was told it's about 500 calories per day vs 300 or so extra prior to birth) the strain on your body is lessened because you are producing a single item (milk) going to one part of your body that's drained periodically.

In contrast, by the third trimester of pregnancy, your total blood volume has increased by approximately 50% (3-4 pounds of additional blood), not to mention an extra 2-3 pounds of retained fluid, 2 pounds of amniotic fluid, and 1.5 pounds of placenta. All told, this is another ~10 pounds on top of the baby. Additionally, the mother / placenta is performing all respiratory and blood filtration functions on behalf of the fetus (the kidneys don't really do much prior to birth other than produce urine).

> https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-...

Personally, while breastfeeding and newborn care was exhausting, it felt more akin to training for an athletic event vs having another entity symbiotically tied to my system like pregnancy.