The Last Guardian may have been too ambitious. That’s why it took so long to develop it and if anything, the install size is evidence of how big the game can be.
The Last Guardian install size is 15 GB on the PS4
Graphics and detailed animations might be why Wccftech estimates The Last Guardian install size to be 15 GB. But it can hardly be called as a game made from the ground up.
When it comes to the graphics, they probably just went back to the original dev assets (textures, sounds, 3d, models etc which are usually an insanely detailed version of what we see in the retail releases of games). And resampled new, more appropriate assets for the PS4. Hardly a rework/”redone from the ground up”. And it would still be pretty much the same game, since the original dev assets they used, would still be the same. Obviously, the game will in general look the exact same, but only with better graphical fidelity.
However, that isn’t to say there aren’t any new features at all. They did design the creature to be stubborn and at times even annoying, to create a feeling of Trico being somewhat sentient, instead of the vehicle of the player. One example of Trico showing somewhat animal behavior, was right after he jumped over the gap, and the player wanted him to catch the boy. He called Trico, and Trico just looked back silently like “what are you on about?”. When the player started to slowly walk away, he only then was like “hey, don’t leave!”, because the boy had helped him before this area.