Why Single sideband transmits only one sideband and suppresses the carrier, so its bandwidth is just the audio bandwidth used, typically about 2.4 to 3 kHz. Full-carrier AM needs both sidebands, roughly twice that. Angle modulation (FM or PM) for voice spreads energy over several kHz of deviation plus sidebands, so a typical narrowband FM voice channel runs around 10 to 16 kHz. That makes SSB the most spectrum-efficient of the phone modes listed.
Watch out Vestigial sideband is the closest contender, but it keeps one full sideband plus a leftover piece of the other (it is used in analog TV video), so it is always wider than true single sideband.
SSB about 3 kHz, AM about 6 kHz, FM about 10-16 kHz. One sideband beats one and a fraction.