After reading threads after threads here, it occurred to me that the term sissy is quite vague in contrast to the term little girl (or transgendered girl). Are sissies boys at heart? Or do they want to be 100% girly? Do they feel awkward and ugly to have boy parts down there (in panties or diapers)? Or do they want to get rid of all boy parts and traits and become girls? The term sissy means literally a man who acts effeminate or dresses girly -- but the key here is the word "man" (or male), right?
Being a girl is nothing humiliating (if you are a genetic girl or transgendered girl), unless you insist on being a man underneath the facade. The idea of sissification often accompanies shame and humiliation.