It's normal. But you must try to stretch the entire length evenly, don't just start pulling at the last 2 feet that miss an inch.
Mark the center of the boot, front and back(long sides), and the center of the sides (short side) , basically dividing them up in quarts. Then mark the rubber accordingly, find a suitable way to do it : wooden plate with 4 nails to make a perfect square, measure with tape, or any other way you prefer. Make 4 marks on the rubber, with equal distance.
Then go and install. Line up one mark with the center mark on the long back side. Clamp it, then stretch to one side to get the second mark (clamp) on the short side and go to the other short side with the third mark; clamp. half of the rubber is now in place, and it's stretched evenly. lining up the forth will finish it off.
as the lip on the body is ot very pronounced, it may slip off occasionally. Clamps and glue are you're friend.
BTW; what are the 2 rubber grommets on the right rear wheel arch? US environmental equipment attachment ?
I haven't noticed them ever before...