Smaller second stages have smaller diaphragms and it is very hard to incorporate a smaller diaphragm into the design without compromising performance.
The diaphragm is essentially a force multiplier that provides force to operate the lever. For a given amount of force needed to operate the lever and open the poppet to get the second stage to begin to provide air, a smaller diaphragm will require more vaccuum which all other things being equal means a higher inhaltion effort. The smaller sized SP regs like the R390, S550, S600 and X650 take diaphragm size about as small as it can go without compromising inhalation effort and even at that, I prefer full sized second stages like the G250 and R190.
The micra is well designed and is exceptional in that when properly adjusted (not always easy to achieve) it has a very good cracking effort, but as noted in the previous post, they are not the smoothest breathing regs around.
Bubble interference is a common problem with smaller second stages, but t is something that bothers some divers and not others and mask designs plays a large part in where the errant bubbles are routed as well.
Reduced jaw fatigue is often cited as a benefit of smaller second stages which are billed as being "lighter". But the lighter issue is debateable as the internal components tend to weight the same and the larger case on a lfull sized second stage will displace more water. So in the water any perceived difference in weight/negative bouyancy is negligible. Mouthpiece design and proper hose lenght have a much bigger impact on the force required to hold a second stage in your mouth bt they are iropnically enough often totally ignored, particularly hose lenght.
Smaller second stages also tend to have a snob appeal factor. People often associate smaller with better and while it is for the most part true with things like cell new and improved cell phones, a second stage is not a cell phone.
