Another commenter has tackled the heart specifically, but on a more general note if your organs get bigger, the volume they displace still has to go somewhere, and there is only so far that the ribs and the chest can bulge to accommodate. So they start pressing and compressing on other organs, which can be dangerous.
As an example, a larger liver is not harmful in itself, but if that causes it to press into your lungs or your guts, then that can cause breathing or digestion problems.
ELI5 version: you have a box that fits exactly 10 golf balls, and the lid fits perfectly. Now try and replace those 10 golf balls with tennis balls and try and get the lid on without squashing anything or pushing the sides or lid out
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