le comparatif des adjectifs

comparative adjectives indicating more or less

Adjectives are frequently used to compare things, people, events, ideas etc. Plus ... que conveys the idea of 'more ... than', moins ... que the idea of 'less than.'

comparative adjectives indicating similarity or equality Aussi ... que conveys the idea of 'as ... as'.

irregular adjectives

The adjectives bon and mauvais have irregular forms of comparison, meilleur and pire. The regular form plus mauvais que has become commonly accepted. 

Note that this irregularity is found in English too with 'better' (not*gooder) and 'worse' (not *badder).