I actually just read about this. Whales are so big because that way they can dive deeper and do it faster.
Its been said that andacondas are larger than their boa brothers and their python cousins because they are aquatic...the larger bodymass is supported better by water---thus, they get bigger...
Water supports larger bodymasses, once whales come on land their mass can collapse lungs or suffocate them...
It was theorized that the large saurapods like Apatasaurus (Brontasaurus), were aquatic due to their large frames and long necks. the theory has been abandon for them to be more like modern day Elephants
those three specimens are unique, as #2 clearly seems to be morphing into some type of Elephant seal---once it has fully morphed, the other two will follow.