It had nothing to do with the power. It's the suspension geometry. Since the rear axle moves in an arc relative to the swing arm axle, which is also offset from the countershaft center, the chain on a swing arm drive is tightest when all three shaft centers are aligned perfectly. Once it swings past alignment in either direction the chain gets more slack.
Dirt bikes have a lot of suspension travel, so in order to keep the chain tension correct at the tightest point in the travel, the slack spec at full droop is much larger since.the rear sprocket is closer to the countershaft due to the arc of travel.
On my dual sport, they state the slack spec as 15mm at the tightest point of the chain with all three shafts in alignment. At full droop, this makes my chain slack 40mm.
A lot of people also confuse the concept of chain "stretch" with the idea that the metal links of the chain are literally elongating. That is not what is happening. Chain "stretch" is caused by roller bushing and pin wear. The pins move more within the rollers, which allows the distance between the rollers of inner and outer links to get closer or farther apart. The pin center to center on each link is the same. If the pin center to center changes on a single link plate, it has suffered a tensilary failure.