There is evidence that Kzinti were tall to 3 meters as you shown, but not evidence of such large Caitians. Or is there?
Only in case of the "they are the same species, just split" theory.
If the two were split so far back in history that there has been time for them to evolve to markedly different heights (read: tens of thousands of years, barring special circumstances), their technology levels would probably be very different from what we see, as splitting would only take place after interstellar travel is invented.
...Unless we postulate that splitting happened long before discovery of spaceflight, and only the smaller or the larger variant went interstellar, leaving the other behind - but that would be putting even more weight on the difference between the two from the get-go, and would not really be part of the "the species are related" theory that e.g. Foster and FASA put forth. That particular theory explicates the "lost colony" concept that does not
allow for tens of thousands of years of differentiation.
If the Kzinti of Trek were the same as the Kzinti of Niven, then of course there would already be great physical differences - the pink batwing ears and rat tails of the Kzinti, as opposed to the general purriness of M'Ress. But "Slaver Weapon" shows us Kzinti kittycats that are no more different from M'Ress than the two felinoids of ST4 are; the funny ears are lost in the generic orangeness of the animation, and we never see an exposed tail.
So, "related" in this particular storytelling sense means "biologically identical", just as with Vulcans and Romulans - a separation of populations for mere thousands of years at most, not enough to separate the species biologically. Save for whatever the species did to themselves, by biotechnological means. (Which do feature heavily in Niven's Kzinti, but not explicitly in the Trek version.)
EDIT: Actually, we do see an exposed Kzinti tail. And it's Caitian rather than ratlike. Although as far as plot logic goes, this picture is a mere coloring error, because there's no real reason for this tail to be exposed: