I hit post before finishing my thought. Bleh. So if the elephant population can change (smaller tusks) in such a short time, there’s no reason to think that a similar change regarding size couldn’t also take place.
There are stories in India that the elephants here used to be bigger than they are now.
Oh, and further, Asian elephants now are bred specifically (when they are deliberately bred) for utility in farming and forestry, which would tend to require a smaller, more “nimble” elephant that is also more tractable. A large war elephant would not have the temperament to move logs all day, and would be more difficult to restrain and would have a harder time navigating the dense forests of Asia. So on top of most of the big war elephants possibly having been all but wiped out in battles, those that were left were probably not bred back into the population due to their now-undesirable tendencies.
I’d bet I could have Asian war elephants that towered over even Savannah elephants within three or four generations again, if I tried.