it doesn't mean you multiply your current stat. it's not quite THAT powerful.
say you have a zigzagoon with tackle and growl. a poocheyena takes out a quarter of zigzagoons hp. now zigzagoon uses growl, lowering poochyena's attack power. now that same move will only do about a fith of zigzagoons maximum hp, or maybe a sixth.
according to what your theory as to how this works, it should have taken only an eighth of zigzagoons maximum hp.
these moves lower the stat of the pokemon by one. if it says sharply rises or harshly falls, that means it is increasing or decreasing by 2 instead of 1. if you look at damage formulas, you'll see that it can make quite the difference. another example:
larvitar uses rockslide on oddish. takes out a quarter of his hp. since i don't have any other move that can do more damage, i use screech 3 times, lowering his defense by 6. it may not seem like much, but after that i used rockslide and took out 3 quarters of his hp.