No. Cage length has nothing to do with capacity - the largest cog a derailleur can handle. Cage length only affects how much chain slack there can be in the drive train.
Originally Posted by
operator
100% false.
Cage length determines the capacity of a derailleur to take up chain slack.
Changing to a wide cluster > 27 *usually* exceeds most short cage RD's capacity.
Bullcookies. I used the wrong term ("capacity' - what is the correct term, anyway?). But I was correct on both points. Max cog size is determined by the parallelogram geometry. Cage length does not determine max cog size, only the ability to take up slack.
It is possible that a setup with a large rear cog will have too much slack for a short cage, but it is also possible that it won't. To the contrary, a very large cog will always exceed the ability of the RD to shift to it.