This resumes what some of you said in previous messages.
Conventional grammars say that feminine adjectives are derived from the
masculine forms. My opinion - and I share it with many scholars - is that it
is the feminine form of the French adjective that is the reference one. The
masculine is obtained by the apocope of the final consonant, that becomes
petite [poe-'tit / ptit] (fem.) > petit [poe-'ti / pti] (masc.) "small"
As pointed out, when the consonant is /n/, it is replaced by the
nasalisation of the vowel.
grande [gra~d] (fem.) > grand [gra~] (masc.) "big"
In the case of /i/, from the phonetic point of view, /i/ is replaced by /E/.