In terms of physical size, I believe the Alien Grunt is bigger, however that's not Bolteh was referring to. He was referring to how the Alien Grunt has more polygons than the Enforcer.
Other than physical size and amount of polygons, both of them are pretty even. Both of them can take a good beating before they go down. I'd go with the Enforcer since they tend to be more of a pain taking them down when in groups than Alien Grunts.
i agree with you in physical size that alien grunt is a bit bbiger and taller than enforcer(remember that the main character in q2 is also a monster for civilian like us. he's about 7 foots in the game makes him too tall too. unlikely for gorden freeman his height is about 6 foots . which freeman is an avarage american). alien grunt is about 7 foots 4 inches height and enforcer is 7 foots 2 inches height
and yet enforcer also have .50cal liquid heat steel chain cannon(that's what it said in quake 2) which made the shield protection in half-life total useless in the field(actually alien grunt dont wear heavily armor like human grunt too. but that just anather case.)
enforcer been more mobilize than alien grunt and the physical melee attack is also painful in the game(in q2 expansion pack enforcer are even tougher.).
in game you are have to take more than 12~15 shot to kill an enforcer with .50cal chain cannon. and alien grunt can be finish with just mp5 in 7~9 shots.
but alien grunt is still kickass because their hornetgun. which can trigger the target in any range. so if in closer combat. enforcer will take advantage but in long range i'd say alien grunt will kill enforcer easily.