If the more gravity there is, the more difficult it is for the heart to pump blood, So that's why creatures that say were big as godzilla couldn't live on earth, because it would have so much pressure on the heart that it would fail.
But what if there was less gravity? The only way to do that would having a smaller planet(Assuming smaller planets would have less mass), then there would be less pressure allowing larger creatures to live, So:
Larger Planet = Smaller Creatures
Smaller Planet = Larger Creatures
But the more you would have to decrease the size of the planet to increase the size of a creature, what if the creature got so big that the planet would end up being uninhabitable for the creature because of it being almost as big as the planet, as well as the creature being so big that it would not have anything to eat and would even pull the planet itself due to its size being almost or even larger than the planet.
So, here are a few questions:
1: What would be the mass/size limit?
2: If the larger the planet the smaller creature would that mean if a planet was big enough the size would be down to atoms and life that requires blood would be impossible?
3: Any Points that disprove this theory?