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.
No you're not.
It's clear you don't understand the difference between max cog size, how it's determined and what it changes. It has nothing to do with "parallelogram geometry". Your misuse of the term capacity is a further indication. Read page 32-21 & 32-22 of the pdf I linked above, in addition to 32-19.