Positive, Comparative, Superlative
This house is big. It is also expensive. ------This house is bigger than the first. It is also more expensive.--------This house is the biggest and the most expensive of all.
Adjectives have got three forms : positive,comparative and superlative.
We use the comparative form+than to compare between two people or things. e.g. : Tom is taller than Richard. Russia is larger than Australia.
We use the superlative form+of/in to compare three or more people or things. e.g. : Chris is the tallest of all.
We use in to talk about places. e.g. : Russia is the largest country in the world.