Here's why:
* Short vowels are typically found in words with one syllable or in words where the vowel is followed by a consonant. The vowel sound in "heavy" is short because it's followed by the consonants "v" and "y".
* Long vowels are typically found in words with two syllables or in words where the vowel is followed by a silent "e".
Therefore, the vowel sound in "heavy" is a short vowel sound similar to the "e" in "pet".