German

big

tall

large

I have a big brother and a little sister

kitchen

kitchenette

kitchen furniture

Gender: feminine

The kitchen is big!

It is a big window made out of glass.

The cat is not small

This horse is larger

They obviously have a lot in common as they both get an -er ending. But please keep in mind the distinction of predicative adjective and attributive adjective:

An adjective is used predicatively in sentences with to be .
Predicatively used adjectives don't get inflected: „Der Hund ist groß.“ / „Die Katze ist groß.“ / „Das Pferd ist groß.“

An attributive adjective is placed in front of a noun and these adjectives receive an inflection depending on the gender, number and case of the noun. And the inflection pattern also depends on the article or possessive determiner in front of it:
„Ein großer Hund verfolgt mich.“ (nominative mixed) – “A large dog chases me.”
„Der große Hund beißt mich.“ (nominative weak) – “The large dog bites me.”
„Ich verjage den großen Hund.“ (accusative weak) – “I chase away the large dog.”

These examples all use the normal positive form of the adjective, but the rule also applies to comparative adjectives. And in principle you turn the positive form of the adjective into the comparative form by adding -er- after the stem – before the gender/number/case ending described above for attributive adjectives.

„Mein Hund ist schnell. Dein Hund ist schneller.“ (both predicative) – “The dog is fast.”
„Mein schneller Hund (positive attributive) und dein noch schnellerer (comparative attributive) Hund verfolgen eine Katze.“ – “My fast dog and your even faster dog chase a cat.”
„Wir suchen meinen schnellen Hund und deinen schnelleren Hund.“ – “We're looking for my fast dog and your faster dog.”

There are some special cases in which it takes more than adding -er- to the adjective to form the comparative. You can read about them here. groß => größer is one of them as the o in the positive adjective turns into an umlaut in the comparative.

„Mein großes Pferd und dein noch größeres Pferd…“ – “My large horse and your even larger horse…”

So if you see an adjective, I suggest you to first check whether it's used predicatively or attributively. If it's used attributively then there has to be an inflection according to the gender etc. And if you still have a spare -er- left after you account for it, its the comparative form. On the other hand, if the adjective is used as a predicate, then an added -er at the end has to be the comparative form as there wouldn't be an inflection according to the gender:

„Ein schneller Hund…“ with attributive adjective. The er is due to the masculine gender, no spare -er- => “A fast dog…”
„Ein Hund ist schneller.“ with predicative adjective. There is no inflection according to gender so the -er at the end has to be due to the comparative form => “A dog is faster…”
„Ein schnellerer Hund…“ with attributive adjective. One er is the result of the inflection but you still have a spare -er-, so this is also comparative => “A faster dog…”

This is a big boat

The kitchen is small

His men are big

bigger

big

large

That is a bigger orange

The coat is bigger

She has a little daughter

He is two meters tall

They have a big house

The apples are small

This is a big strawberry

at the

atthe

onthe one who

inthis

We're eating at the big table

She has a small dog

plate

plates

Gender: masculine

Our big plate is green

I have a big cat

My cat is the larger cat.

You have big brothers and big sisters

motorcycle

Gender: neuter

Do you have a big motorcycle?

The men are big.

at the

at

the

We're eating at the big table.

Everyone has a big window.