Thank you reyalp, SX720, and Joker135 for the education. As Joker135 mentioned that the letters, L, M, and S set the resolution of the image as I have expected. In various older models of Powershot cameras that I owned L=highest image quality, M=medium image quality, and S=lowest image quality. With the SX740, I found the quality is the same for L, M or S. I was just disappointed that my 10 years old SX110 at 9 megapixels seems to have a better image than the SX740. I also borrowed a friend's SX730 (160dpi) with Digic 6 and found the image was also better than the SX740. Perhaps it is the limitation of Digic 8 to support 4K video that something else has to give.
Again, thank you for your comments.