The chain is stretching because it's new. A new saw chain must be retensioned more frequently. Check tension with the chain cold and the engine off.
Fix: Correct tension is when the chain fits snugly against the underside of the bar but you can still pull it along the bar by hand. Use the side or quick chain tensioner to adjust. Run the saw at idle for a minute to spread oil, then recheck. You will need to do this several times during the first tank of fuel.
If that didn't work: The guide bar groove may be worn out. Check the groove depth. For a 3/8" pitch chain like yours, the minimum groove depth is 6.0 mm. If it's less, replace the guide bar. Also, regularly clean the oil inlet hole, the oilway, and the bar groove to ensure proper lubrication and reduce friction.
Heads up: After a long period of full-throttle operation, let the engine idle to cool down. A hot chain stretches more. And always slacken the chain when you're done if you retensioned it while it was hot.