Being a science graduate myself, I’m familiar with the physics of such a size difference. What I like about writing, especially in a universe with a supernatural bent, is that we can take a lot of those rules and bend them.
Sam’s size alone would be difficult to maintain for such a long lifespan. They should have higher metabolisms, age quicker, eat a lot more for the energy needed for this. Falling would do less damage, it’s like when you drop an elephant, it splats, when you drop a mouse, it bounces. (ffs who comes up with these experiment ideas; protect the elephants)
Especially in the extra tiny Jacob AU we do step back from those rules. He won’t have to worry about the water sticking to him that much. Sam can hurt his wrist when he falls just like Dean can, and they can (mostly) communicate normally.
(Jacob has some issues talking to Dean in the horror story, as we’ll soon see. He is too smol even with our lenient rules.)
We did have a previous ask come in about this like a year ago, so if you want to read some more into our thoughts, check it out here!