Why is it "rōvyzy" here instead of "rōvyz"?
Because it does not act as an attribute (adjective modifying a noun, not through a verb). It would also be the form of the adjective when after the noun it modifies. "big eyes=rōvyz laehossa=laehossa rōvyzy [emphatic]"
Oh yeah, I vaguely remember reading something about pre- vs post- position of adjectives. I suppose I'll have to go back and read that again.