This is the 2981st most frequent Catalan word.
gros
Large; big.
In this sentence, 'gros' is used as an adjective to describe the size of the dog, signifying that the dog is large.
El gos gros descansava sota l'arbre.
The big dog was resting under the tree.
Here, 'gros' again describes size, indicating that the fish observed was considerably large in size.
Vaig veure un peix gros al llac.
I saw a big fish in the lake.
In this usage, 'gros' is applied metaphorically to convey the importance or seriousness of the problem, rather than physical size.