Increased Cylinder says it turns the ammo from 6 to 8. Basically it's the Extended Clip of Cylinder-guns (+25% of 6, rounded up, means +2). If you expand it to include revolvers that don't use a 6-round cylinder, you can grab that +25%, rounded up, so 5->6.25->7.
Alternatives are "it always adds 2 rounds" and "it adds 8/6, rounded up", but I think it's just a different-named Extended Clip mod. As support for that explanation: Their stats are identical, aside from the Threshold. Same slot-space, same nuyen cost, same availability, they only raised the threshold because revolvers are special.
That means only a revolver with 9+ rounds would get +3 from this btw.