'Zackly. If you feed a "WIDE" stereo signal in, then the 'S' (difference) output may well be larger than the 'M' (sum_ signal.
Incidentally, this is how it's done in FM broadcast to remain mono-compatible. The stereo signal is first filtered with a brickwall filter at 15kHz, then split into L+R (mono, 'Middle', or sum) and L-R (difference or 'Side") and the L+R mono signal is broadcast just as in the days before stereo radio, so if you have a mono bedside radio, it receives the mono and stereo stations in mono... no problem with compatibility.
The L-R (difference signal) is lower sideband encoded onto a 38kHz carrier. The carrier is out of the hearing range, and is attenuated significantly by old radios, sue to the pre-emphasis and de-emphasis that was always part of the mono FM standard anyhow, so luckily helps out. (like your old 1930s radio speaker will reproduce 38kHz anyhow!) Since the difference signal never has any frequency in it which is higher than 15kHz, the lowest it will swing the LSB is 38kHz minus 15kHz, so 23kHz. -theoreticaly inausible, and only when there's significant HF content in the signal, when it would be masked by the HF content in the 15kHz region anyway...
Anyhow, the stereo radio locates the 38kHz carrier, lights a red "STEREO" light to indicate the prescence of the carrier indicatinfg a stereo transmission, surpresses it, decodes the LSB difference signal and adds and subtracts it.
(L+R) + (L-R) = 2L
(L=R) - (L-R) = 2R
Violà! -Stereo!
Keef