Ese libro es más grueso que el mío.

Because más ... que is the standard pattern for making comparisons of greater degree in Spanish.

• más = more
• que = than

So:

• más grueso que = thicker than

This works with many adjectives:

• más alto que = taller than
• más caro que = more expensive than
• más rápido que = faster than

In this sentence, Ese libro es más grueso que el mío, the structure is:

• Ese libro = that book
• es = is
• más grueso = thicker
• que el mío = than mine

Both words can relate to the idea of something being thick or fat, but they are used differently.

• grueso is the normal word for something physically thick in size or width, like a book, wall, rope, or sweater.
• gordo usually means fat or overweight, especially for people or animals, though it can sometimes describe objects in certain contexts.

For a book, grueso is the natural choice:

• un libro grueso = a thick book

Using gordo for a book would sound odd in most normal learner contexts.

Because mío is being used here as a possessive pronoun, not a possessive adjective.

Compare:

• mi libro = my book
  
  • Here mi goes directly before a noun.
• el mío = mine
  
  • Here the noun libro is omitted, so Spanish normally uses the article el with the possessive pronoun.

So:

• Ese libro es más grueso que mi libro = That book is thicker than my book.
• Ese libro es más grueso que el mío = That book is thicker than mine.

Spanish usually needs the article in this kind of structure:

• el mío, la mía, los míos, las mías

The accent mark shows the stress and helps keep the pronunciation clear.

• mío is pronounced in two syllables: MÍ-o
• The accent tells you the stress falls on the í

Without the accent, Spanish spelling rules would suggest a different stress pattern, and it would no longer be the correct written form.

This accent appears in the possessive forms:

• mío
• mía
• míos
• mías

Because el mío refers to a specific masculine singular noun that is understood from context: libro.

• libro is masculine singular
• so the possessive pronoun must match it: el mío

Use of lo mío is different. It usually means something more general or abstract, such as:

• Lo mío es la música. = Music is my thing.
• No toques lo mío. = Don’t touch my stuff / what’s mine.

In your sentence, we are talking about my book, so it must be:

• el mío

Because possessive pronouns in Spanish agree with the thing possessed, not with the owner.

The possessed thing here is libro, which is:

• masculine
• singular

So the correct form is:

• el mío

Other possibilities would be:

• la mía for a feminine singular noun
• los míos for a masculine plural noun
• las mías for a feminine plural noun

Examples:

• Tu casa es más grande que la mía. = Your house is bigger than mine.
• Esos zapatos son más caros que los míos. = Those shoes are more expensive than mine.

Both are demonstratives, but they show different distance relationships.

In standard Spanish from Spain:

• este = this, near the speaker
• ese = that, near the listener or a bit farther away
• aquel = that over there, farther from both

So ese libro means that book rather than this book.

A learner should remember that Spanish often distinguishes three levels of distance, where English usually uses mainly two (this / that).

Es is the third person singular form of ser in the present tense.

It is used because the subject is Ese libro, which is:

• third person
• singular

So:

• Ese libro es... = That book is...

The verb ser is used here because you are describing a characteristic of the book: its thickness.

Normally, no. In this sentence, ser is the correct verb.

Use ser for describing an inherent or identifying characteristic:

• El libro es grueso. = The book is thick.

Using estar with an adjective often suggests a temporary state or a result, and with grueso for a book it would usually sound unnatural.

So for a normal comparison of physical thickness:

• Ese libro es más grueso que el mío ✅

Yes, if the context already makes it obvious that you are talking about a book.

Spanish often omits a noun when it is clear from context:

• Ese es más grueso que el mío. = That one is thicker than mine.

This is natural in conversation when both speakers already know what object is being discussed.

But if there is no clear context, keeping libro makes the sentence clearer.

Because Spanish does not need an extra word like English sometimes does.

In English, we often say:

• my one ❌
• mine ✅

Spanish behaves similarly here:

• el mío already means mine / my one in context

So Spanish does not need another word. The noun is simply understood:

• el mío = my book

In most normal situations, it would sound less natural than más grueso.

A native speaker would usually describe a book as:

• grueso = thick

Using gordo for an object can sometimes happen in informal or expressive speech, but for a learner, grueso is the safer and more standard choice for a book.

So the best option is:

• Ese libro es más grueso que el mío.

Usually, no. After más, Spanish normally uses que:

• más grueso que
• más alto que
• más interesante que

However, you may see de after más in expressions involving numbers or quantities, for example:

• más de cien páginas = more than a hundred pages

So:

• más grueso que el mío = comparison between two things
• más de cien páginas = quantity, not this kind of comparison

That is why your sentence uses que, not de.