It's likely they are stopping peon production at 9 peons also.. which means they can instead spend the resources on grunts early .. so the money you spend on peons and extra farms to support those peons, they're spending strictly on grunts (9 peons, 4 farms) .. it's a really aggressive strategy and works if the other person doesn't know how to respond.
If your enemy is "p stopping " (9 peon stopping) it should be obvious "omfg how do they have so many grunts" .. that's when you need to play defensively, form a choke with your grunts and force them to try to break the choke.. the trick is to keep your choke in tact while you continue peon production for the most part.. if you can hold the choke, you will demolish them mid-late game because you will be so far ahead in peons which = resources.
If the choke breaks ..AS SOON AS IT BREAKS pull your grunts back to your peons on gold, and start using peons to attack as well. It takes unit control to do properly but if that choke breaks, using peons to assist in fighting temporarily is your only hope.. and try not to lose any peons to their grunts (unit control...pull peons that are getting hit out of the battle)
Also.. there may come a time where you need to stop peon production temporarily and spend the rest on grunts yourself.. there are clues that you will start to see that give u indications on when to be aggressive, defensive, focus on peons, focus on peons etc..
