I don't mind the size, I mind buying batteries all over again. Yes, they last a little longer, but mods are considerably bigger. And one of the fears is of the 26650 like flop. They became so popular, yet sag on 26650 was so pronounced they were useless.
That too is understandable. But with mooch's tests, his charts show that some of these 2x700s are true performers. His charts show the sag is very low with some of these cells, but offer longer run time and higher amp limits. With less sag, more mah, and more amps, it is hard to deny that these batteries very well look like they may become the new 18650s over time.
I started back up with vaping at a good time, coincidentally. These cells were just gaining traction when I needed to restock on new batteries. If I were already heavily invested in 18650s, I would most likely not have bought any 2x700s yet either.
And, personally, I couldn't say 3mm x 5mm more is considerably bigger for a mod. Noticable, but not a huge difference in size.