I think it's fine to lump fungi together with plants for the spirit of this discussion. The distinction between plants and fungi are made on the basis of metabolism but in this thread people are clearly talking about external behaviours observable with the naked eye.
Metabolism influences external attributes (I think behavior isn't the right word you're looking for). A larger creature requires higher caloric intake than a smaller creature (and this process isn't linear). A larger creature has different thermal regulations than smaller creatures because surface doesn't scale with volume linearly. The bigger the creature the more it calories it has to expend on the body vs the mind. There are equilibriums here that are physics based.
And then consider the selective pressures from plants vs fungi. A fungi gets its environment. Plants do some, but also need to perform photosynthesis. These have very different selective pressures for these lifeforms.
That's not true, we also have fungi as a macroscopic lineage.
Or maybe you put them in the plant group? If so, that's a mistake, as they are more closely related to animals than they are to plants.