Why SSB transmits only one sideband and suppresses the carrier, so a voice signal occupies roughly 3 kHz. FM voice spreads the same audio over about 10 to 15 kHz because the deviation and sidebands widen the signal. That narrower bandwidth is why SSB concentrates transmitter power into a smaller slice of spectrum, making it the mode of choice for weak-signal HF and VHF work.
Watch out The claim that SSB is easier to tune is backwards: SSB needs tuning within about 50 Hz or voices sound like ducks, while FM tunes in easily anywhere in the channel. SSB is also more prone to noise and interference, since FM's limiter strips off amplitude noise.
SSB about 3 kHz, FM about 10 to 15 kHz. Narrow and fussy to tune versus wide and forgiving.