It will definitely stretch with the weight of the baby. I had a smaller size and exchanged it for the larger size. Wish I would have kept the smaller size because it stretches a bit. I loved my Baby K'tan though for my dd when she was little. After you use it a bit you get used to how to put it on. I had to look at the directions the first few times.
If you went by the guidelines then you probably have the right size. It sure streeeetches! Practice with a stuffed animal or something. It would not be the same weight of course but you'll get a better idea of how to get it on and off easily. I used to sit down to put mine on too when LO was tiny.
Re: Baby Ktan
Photo taken at 16 months old