It’s all based on the design objectives – if the objective is to keep the device as small as possible, then it is only logical to utilize a smaller cell. Some may view this as a limitation, but that will be based on their own vaping characteristics. Others may put importance toward size and are willing to sacrifice a bit of run time in order to meet that value.
Personally, I agree with both sides of the debate. My own vaping style does not suit a single 18650 device for it is difficult to get a full day of use given the power I typically use. But that said, I also own 2x700 devices that I rarely use because I feel they are just too large. I’m ok using these larger devices at home, but never use them when I am out….is that a dual 21700 mod in your pocket or are you just happy to see me