Hm, strange. What kind of textures does it use? Custom or ingame? Yes, this cylinder and sphere make the filesize go up that much.
I know it looks better this way, but you should change it, otherwise it will be rejected.
As for the error just try this way: open it in MDLVIS, and got in the menu on top to ''Optmize'' and save it. Then delete the old version from WC3 Editor, save the map and re-import it. IMPORTANT: the model should be imported as ''Waterpump.mdx'', NOT .mdl. But my main guess is that it has to do with the filesize. I don't want to insult you, but not even the most detailed models (like Frankster's tanks and -Grendel's exotic creatures) have such a filesize and they have an entire load of different features and they're very detailed too. Maybe 21540 vertices are much more than WC3 can handle.
Take the War Bear Tank model for example. It has 5018 vertices (poly counts) and a filesize of 438.64 KB and this is already quite much for a single model. Yes, it does have many features, high detail lvl and such, but your water pump has more than 3 times the vertice amount of this and the filesize is also nearly doubled. I dunno what features and such it offers, but it is way to much of both. I know models with this detail lvl look better, but it's possible that the editor and game can't handle so much. This kind of Poly count and filesize are mainly for games like Far Cry, Alien vs. Predator 2010, Call of Duty and such, i.e. high-resolution, super-realistic FPS & Ego-Shooters. Trust me, my first models were made the same way and i can tell you it just is unnecessary to make such a shape-perfection for a strategy game, like WC3 as the majority of details won't be visible ingames anyway.