I just checked out the
FSUSA webpages for these: $80 for the 400 mAh
Jessy S (the one you reviewed, right?) ... $90 for the 650 mAh
Jessy M ... and $110 for the 900 mAh
Jessy X. Indeed, the M and X have three "atomization level" settings: 3.5V, 3.6V, and 3.7V. The S just has 3.5V and 3.7V. The M appears to be just fatter than the S; the X appears to be longer and fatter.
Rather pricey for fat batts, but if those subtly different voltage settings
do deliver two or three markedly different
vaping experiences, it may be worth it.