Not only kanji, but also hiragana and katakana (syllabic alphabets) encode to three bytes per character. Shift-JIS can encode all three to two bytes, as well as half-width katakana to one byte per character.

However, if size is such a concern (eg for web transmission), text compression neutralizes the perceived benefit of region-specific encodings.

Shift-JIS' continued popularity has much more to do with change aversion than it does technical merit.