There is very little that can be answered here. Post-mortum changes can occur after
death, but significant heart enlargement likely occurs over time, prior to death, but the amount of time can be variable, depending upon the circumstances. Children can have structural deformities of the chest called "pectus" and it can be a "sunken" chest (pectus excavatum) or a protruding or "pigeon chest" (pectus carinatum).