Why ocean animals are big in size ??

 It comes down to physics, food and other limits.

1.  Water supports weight

                    On land , we have gravity.  An animal's skeleton and muscles have to hold it up 24*7 against gravity.

                      But in water, buoyancy cancels most of this out.

                      A blue whale weighs 200 tons, but in water it feels almost weightless   !!!

                       This allows sea animals to grow bigger without breaking their bones.

2.  More food, more space

                     In oceans, there is huge uninterrupted food chains. There is nothing to stop a predator from roaming to find it.

3.  Heat management is easier 

                      Big bodies heat up and cool down slowly. In water, heat dissipate faster .

                       So giant marine animals don't overheat  !!!

4.  Less predation pressure 

                        In open ocean, size itself is the best defense.  For big marine animals, there is nothing that can kill them !!!