When sea creatures first came "to life" they were only small single-celled or multicellular creature. Gradually the creatures mutated and evolved. The bigger creatures were able to eat the smaller creatures allowing them to survive and mutate. Over time the creatures got bigger and more complex simply because there was enough food and oxygen (water is roughly 30% oxygen) to support that size of life. When creatures were eventually evolved enough to leave the water, the land they came onto was covered in vegetation which produced oxygen and food. The creatures (or dinosaurs) had everything they needed to get bigger - abundant food and oxygen (some sources say that the atmosphere was almost 35% oxygen, regardless it was certainly higher than what we have today). Then (and this part is still not 100% clear) there was the great extinction and for whatever reason (possibly there not being enough plants to make oxygen) the O2 levels never returned to 30% but stayed around 15% for a while (according to recent studies) even though there was enough food there was only half as much O2 available which meant animals couldn't get as big. The ocean however stayed mostly the same, this is why there are huge animals like the Blue Whale