There's no such word as biggers in the English language. The correct comparative form of big is bigger, pronounced /ˈbɪɡər/. It's important to understand IPA (International Phonetic Alphabet) symbols to understand how to pronounce English words correctly, as the spelling of English words can be ambiguous. The symbol /ɪ/ stands for the 'short i' sound as in 'it', and /ə/ is the 'schwa' sound, the most common vowel sound in English, pronounced with the tongue relaxed in the center of the mouth.