How "squishy" a snake is, quite obviously, is a rather subjective matter. If your vet says its fine, I wouldn't worry. One of my Borneo STPs has never constricted her prey in the time I've had her--since she was five months old. Another, that I purchased as yearling, does or doesn't constrict seemingly at whim. Its pretty common--in the wild, many boids will chow down on dead stuff too, and some seem to distinguish between the two rather easily. My larger red (misnomer in this case) blood is a very, very, strong snake, but I would never know it from holding her--she will slip out of my hands, off the chair, etc. In the wild, it is unlikely that a blood or STP would ever find itself off the ground. You can tell her strength when she decides to thrash around though (rarely, when she gets spooked). Bloods are frequently a little overweight in captivity, but other than that, I doubt yours has any problems. As to "internal parasites," be they bane that they are, they don't result in "squishy" snakes of good body-weight, they result in sick and/or emaciated snakes. In general, if its feeding, things are going to be alright.