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Well yeah, primary factors affecting the size of the fruiting body of mushrooms are optimal temperature ranges (usually temperate, and humid) access to substrate containing high levels of carbon and nitrogen as well as lignocellulosic substrate, such as that found in shrub-land or from agricultural runoff.

The main factor that would limit the sustainability of a mushroom forest would be the density and area covered by mycelium, as to support the amount of nutrients needed to sustain large stipes and fruiting bodies.

The reasoning for why mushrooms were so large prehistorically, or at least some, was because those mushrooms evolved to aerate. In particular cycling oxygen in and carbon dioxide out using large plate-like caps to function as lungs with large surface areas utilising the atmosphere's high oxygen content to aid the metabolic process. And the process by which they broke up rocks into soil was entirely physical being that chitinous fungal bodies spored out of the channels eroded into large stone due to consistent rainfall.

The hand-waving part that chuubanite would be used for is to overcome the mycelium restrictions and allow them to more easily gather nutrients as well as performing the role of a metabolic aid.