I was mostly referring to the end-size rather than growth rates. They seem to be getting larger in my low-flow, low-light tank than they ever did in my high-flow, high-light larger tank. But the flow may be the more deciding factor.
I have witnessed faster growth, though, in dirtier water. When my skimmer broke for about 3 weeks before I replaced it, my red mushrooms (which were not growing before then) took off and cover a good 1/6 of the back of my tank with rows of small mushrooms (and scattered them everywhere to be a plague of the nooks and crannies in my tank). Nitrates got up over 30 at that time. They stopped spreading after nitrates went back down to 0.