Gros vs Grand
How come that when a person, an animal or even a book is big, we say “gros” (Le livre est gros), and the word “grand” means “tall”, but when we want to say “the zoo is big” we’d say “Le zoo est grand”?
To try and make it simple, maybe you can consider that "gros" is a matter of volume (thickness, weight) and "grand" a matter of size (height, length).
Now, a few trends:
• human beings: il est gros (fat), il est grand (tall), c'est un grand homme (great).
Intentionally, "grand" is more appreciative than "gros": c'est un gros commerçant (making money); c'est un grand industriel (respectable).
• animals: un gros chien (contrary of "petit chien", so rather big); un grand chien (tall and slim)
• real things: gros manteau (thick/heavy), grosses chaussures (big/heavy), grande robe (long), grande avenue(wide), grand vin (great wine).
• concepts: un grand rire (big), un gros or grand chagrin (deep/great)