Notice the wrinkles and puckers in the first photo. I stretched the raw canvas reasonably tightly, but it still sags and pulls on the staples. I misted it with water a few times to start tightening it up. Its just like your jeans shrinking in the wash. As you can see, it doesn't take long for the acrylic gesso to really tighten things up. I was reminded of tightening window plastic with a hair dryer.
Proper canvas stretching technique has one starting at the middle and gradually, evenly working to the edges (this is true for stapling as well as priming). This first coat was diluted so it would soak into the fibers and make a good bond. I'll add two more coats full strength to make a strong, non-absorbent ground.