The thing to remember with Kandra is that they are still living beings that take in material and excrete it. In Hero of Ages, we see TenSoon imprisoned and fairly emaciated. A kandra can increase their mass by eating a person or living being and decrease it by expelling the matter.
Is there a WoB detailing the exact degree of control the Stormfather has over Highstorms that I'm unaware of? To my knowledge, his ability to control the Highstorms extends to sending them out. That's all we have evidence for his control over them as far as I'm aware. Not to mention that Highstorms by definition are unidirectional storms filled with investiture that lose their intensity the further west they go, presumably due to losing momentum and such by passing all the mountains on Roshar.
Now, before I go off to look for WoB's to create a decent argument for this thread, I have some questions regarding the situation we're discussing as the answers could be vastly change the match-up. So, what time are we discussing, Right around The Way of Kings and Words of Radiance or something properly in Era 2 Scadrial? By my understanding, trying to match Roshar's capabilities anywhere near the Wax and Wayne novels is an exercise of pure speculation while we have some knowledge of what Scadrial was like roughly around the current Stormlight Archive novels. Secondly, what planet would this war take place on? This is, of course, ignoring any issues with worldhopping armies/scouts/supplies/etc. since neither planet has the capability to do any mass worldhopping by their respective latest books. Thirdly, who are the actual combatants? Scadrial most likely has 6 distinct polities (the Elendel Basin counting as one and counting the 5 peoples in South Scadrial as 5 separate polities) while Roshar has many but we only know a good deal about the military of, by a generous estimate, one to three (Jah Keved doesn't have much fight left after its civil war or during it and Kharbranth is a small city-state which most likely has more surgeons/healers than soldiers).
Sorry for the large block of text. I just feel like the discussion could be better served if we all agree on the actual situation the hypothetical war would take place, something that I've seen has been a problem in the last 11 pages.