A detail in Duty Calls that I think says a lot about Ciaphas Cain’s inner workings is that Cain says when he rides transport for the Astartes (I forgot the specific vehicle type), he always feels a bit self-conscious because the seats are too tall for him.
But, in earlier volumes Amberley noted Ciaphas was just under 2 metres tall (like volleyball player height for other sports nerds)! He knows he always sticks out like a sore thumb when he’s with normal humans! Yet he still can’t help but compare himself to the genetically-altered warriors specifically known for their sizes.
However, what works so well for Cain is that he never makes his self-doubt a problem for others nor does he express it in a destructive way. Most notably his anxiety seems to make him more attentive to others. It’s very sweet that he not only understands Jurgen better than others and accommodates the way he works, he also always keeps note of how Jurgen’s doing wherever they go to make sure he’s comfortable.