Hiya
I never understood the whole concept of 'surface area:volume ratio' until my teacher explained it using a pile of text books; if you take a pile of text books and stack them on top of each other, then take half off, you can see that while the volume halves, the surface area decreases far less dramatically because it is only the surface area of the sides of the books that decreases.
I'm not sure if that makes sense, I'm a visual learner so I'd say it's best to physically get the books and have a look at it working for yourself if you think that would help you. In the case of animals, a small surface area:volume ratio reduces heat loss because there's less skin (area) exposed around the animal, so a smaller area of skin over which heat can be lost from
via radiation. Conversely, a large surface area:volume ratio increases heat loss because there's more skin (area) exposed around the animal, in relation to its size, so there's a larger amount of skin (surface area) by which heat can be lost via radiation.
For example: the elephant. If an elephant had no flaps of skin, it would have the same volume because it would be the same size, but a smaller surface area because there would be less skin around it. Therefore, less heat would be lost via radiation if the elephant had no flaps of skin because,
for the same size elephant, less heat is lost from the skin through radiation.
I hope that's helpful, if it's not then I can try and re-word it or find a visualisation. It all links to the amount of heat that an be lost by radiation in animals.
As for the plants in rainforests; I am aware that leaves in rainforests are often thick and shiny to protect from extreme sunlight, however I'm unsure with regards to rain. Leaves coated in grease take in far less water, so I assume it would work in a similar way and attempt to protect the leaf from absorbing too much water. Likewise, I think the pointed tips are designed so that water drips off the leaf quickly after rainfall, which might aid in allowing more sunlight to be absorbed, as that is more in demand that rainfall in the rainforest.
However that is mostly speculation, so don't take my word for it! Hope that helps