I assume you're referring to the use of a .21 in a truggy over a .28. Anything over a .21 is not legal in a buggy for any class but "sportsman" at club level.

The reason is very simple. Fuel economy. When you get to the upper levels of truggy racing winning comes down to missing a jump, a bad marshall, or a fuel stop. If I can run a 15 minute main and only pit once because I'm running a RB Concepts C6BB .21 with their 2045 header/pipe combo and get 9 mins on a 150 ml tank then I only need to pit once. While the guy running a cheap .28 engine can only get 6 1/2 mins and needs to pit twice. That 10 second pit stop may be the difference between winning and losing.