Just wondering if anyone has used battery sizes other than 18650s (or lipos) in their portables. I was thinking of using 4 21700s (LG) in series for my pack to get 14.8 V 5000 mAh. Now, while my current pack is by no means good or power dense (2000 mAh/cell, 16 cells series), using these batteries would be about 60% capacity for a little over a quarter of the size. This seems much more power dense than even a 3500 mAh 18650s. Why aren't more people using these in their portables, is it a size issue?