Join Date: Nov 2007
Location: NYC & Long Island, NY
Not sure about bulbs, but many plants develop emersed growth and will just start growing right out of your tank if they run out of room below the water line--this is primarily true with stem plants.
I have a 5 gallon hex with some corkscrew vals, and they're consumed practically every inch of substrate, and now they're growing on top of each other, with runners criss-crossing everywhere. Small tank, but no growth slow down or stunting.
I suspect you may get stunting if the plant gets so large in comparison to the tank that it's using up all the available nutrients and CO2. Especially if you're not dosing ferts or CO2.
But I have no direct experience with aponogetons, onions, or other bulbs, so my experience may not help you...