I think you're right. My aztecus is larger than my two cubensis.I know one of them is bigger than the other...I think aztecus is the larger one, and I think it's also larger than mexicanus, which gets up to 22mm. I think cubensis is smaller than mexicanus, as well. However, I am very unsure about that memory, it's based entirely on one picture of three adults that was supposed to be about standard for sizes for each species, and I may be misremembering which species were represented and which was which.
That may be, but if so, it's for the wrong reason. I found the original picture, and it was actually Paraphrynus raptator, P. mexicanus, and Phrynus marginemaculatus.I think you're right. My aztecus is larger than my two cubensis.