  The growing database of gravitational-wave (GW) detections with the binary
black holes (BHs) merging in the distant Universe contains subtle insights into
their formation scenarios. One of the puzzling properties of detected GW
sources is the possible (anti)correlation between mass ratio q of BH-BH
binaries and their effective spin. We use rapid binary evolution models to
demonstrate that the isolated binary evolution followed by efficient tidal
spin-up of stripped helium core produces a similar pattern in Xeff vs q
distributions of BH-BH mergers. In our models, the progenitors of unequal BH-BH
systems in the stable mass transfer formation scenario are more likely to
efficiently shrink their orbits during the second Roche-lobe overflow than the
binaries that evolve into nearly equal-mass component systems. This makes it
easier for unequal-mass progenitors to enter the tidal spin-up regime and later
merge due to GW emission. Our results are, however, sensitive to some input
assumptions, especially, the stability of mass transfer and the angular
momentum loss during non-conservative mass transfer. We note that mass transfer
prescriptions widely adopted in rapid codes favor the formation of BH-BH merger
progenitors with unequal masses and moderate separations. We compare our
results with detailed stellar model grids and find reasonable agreement after
appropriate calibration of the physics models. We anticipate that future
detections of unequal-mass BH-BH mergers could provide valuable constraints on
the role of the stable mass transfer formation channel. A significant fraction
of BH-BH detections with mass ratio q in range (0.4 - 0.7) would be consistent
with the mass ratio reversal scenario during the first, relatively conservative
mass transfer, and a non-enhanced angular momentum loss during the second,
highly non-conservative mass transfer phase.
