  We study the gas kinematics of a sample of six isolated gas-rich low surface
brightness galaxies, of the class called ultra-diffuse galaxies (UDGs). These
galaxies have recently been shown to be outliers from the baryonic Tully-Fisher
relation (BTFR), as they rotate much slower than expected given their baryonic
mass, and to have baryon fractions similar to the cosmological mean. By means
of a 3D kinematic modelling fitting technique, we show that the HI in our UDGs
is distributed in "thin" regularly rotating discs and we determine their
rotation velocity and gas velocity dispersion. We revisit the BTFR adding
galaxies from other studies. We find a previously unknown trend between the
deviation from the BTFR and the disc scale length valid for dwarf galaxies with
circular speeds < 45 km/s, with our UDGs being at the extreme end. Based on our
findings, we suggest that the high baryon fractions of our UDGs may originate
due to the fact that they have experienced weak stellar feedback, likely due to
their low star formation rate surface densities, and as a result they did not
eject significant amounts of gas out of their discs. At the same time, we find
indications that our UDGs may have higher-than-average stellar specific angular
momentum, which can explain their large optical scale lengths.
