I agree with the point of Robby being great and a WW but I wouldn't call him a small WW, in fact I'd class him as a big WW personally.
In a sort of pfp order Armstrong Keed Maxim Zivic LaMotta Fulmer Basillio Olson maybe? And no, the names he didn't fight are more impressive to me than the ones he did. But to his credit, he did fight everyone but Burley at WW and MW. With 4 of the mentioned names being LHWs.
In 1942, two years after turning pro, Robinson was fighting at around 142, 143, and 144 in the 147 pound division. He didnt start weighing at the 147 limit consistently until after his stint in the army which also saw him inactive from the ring for over a year. A year earlier he was fighting at 135. Bivins, Marshall, Moore, and Charles had all fought 20 pounds (at least) north of Robinson by the time Robinson started weighing 143/4. I say again, the fact that people question why Robinson didnt fight these guys is more a testament to his greatness than a stain upon his honor. Nobody ever really questions why a huge middleweight like Monzon, who ducked his number one contender for three years, never dipped his toe into 175. Likewise Hagler. I dont see people criticizing Emile Griffith for not fighting Bob Foster (which is what this comparison is more akin to). When Robinson went into the middleweight division on occasion and fought guys like LaMotta its gravy on his record. He didnt have to fight ANYONE at middleweight while he was a welterweight. He was the biggest name south of Joe Louis and could have easily lived at 147 and cherry picked guys from 135 at catchweights for years to come. Yet more is always asked of him. Thats how great he was.