What is the correct adjective for weight/mass of objects, heavy or large?

Solution 1:

I often see in scientific literature the adjective "large" associated to mass/weight, such as "this animal is larger than another"

Are you sure this is what you see? I would be surprised. The default meaning for "large" when it comes to animals and most physical objects refers to overall volume (of the envelope), not to mass/weight. Of course a high volume object is often heavier than a small object but this is not inevitable.

In the following fanciful image, the balloon is larger than the elephant, even though we know that the elephant is heavier/more massive.

*envelope = a surface that surrounds the shape.