I don't know why Sigma's chin shouldn't change shape, everything else about him changes!
Here's the rest of the captions you asked for:
p.183 Yoshikawa: X, very earnest and powerful, in a head-on confrontation. Zero, looking solemn, tracing a beautiful arc with the sword. And Axl, firing like mad all over the place. We drew these with the intention of conveying the three heroes’ individuality as straightforwardly as possible.
p.218 top Even after the X series was well on its way, the popularity of the original Rockman didn’t decline, and sequels continued to be released.
Sketch from the Rockman 7 period.
middle Inafune: Drawing pictures for both at the same time, the design lines for regular Rockman and the X series gradually got mixed up (laughs).
[Trans. Note: "Haha, wait for future releases" would have been GOOD, it would have meant there'd BE future releases!]