Here's why:
* Short vowel sounds are typically found in words with one syllable and a closed syllable structure (a vowel followed by a consonant). In "big," the vowel "i" is followed by the consonant "g," making it a closed syllable.
* Long vowel sounds are often found in words with two syllables or with a vowel followed by a silent "e" at the end.
Therefore, the "i" in "big" has a short sound, similar to the "i" in "pig" or "dig."