There should be an upper and lower limit as to what levels a creature can scale; a radroach should never gain the HP of a Behemoth, and likewise a Behemoth should always be a threat (And it would be if it wasn't too big to move in the ruins). That is hopefully something Bethesda includes in Skyrim's level scaling. They did IIRC say that dungeons have upper and lower level scale limits (Dragon Age used this system too), so hopefully they include that for wilderness creatures also.