The physical origin of subwavelength photonic nanojet in specular-reflection mode (s-PNJ) is theoretically considered. This specific type of photonic nanojet (PNJ) emerges upon double focusing of a plane optical wave by a transparent dielectric microparticle located near a flat mirror. For the first time, to the best of our knowledge, we report a unique property of s-PNJ for increasing its focal length and intensity using circular-shaped microparticles with a refractive index ratio exceeding the known limiting value that fundamentally distinguishes s-PNJ from regular PNJ behavior. This may drastically increase the trapping potential of PNJ-based optical tweezers.