The most recent entries in the series have featured a worrying amount of enormous, physically impossible creatures that defy any biological law you could possibly think of. In the early days, the big beasties (the Tyrants) were grown in labs from live humans, but the implication is that the process took a long time with the mutation taking weeks at the very least. RE2 did things slightly differently with William Birkin's transformation taking hours, but again being gradual, he evolved slightly every time you saw him. Even then, he was still humanoid shaped (until the end) and only marginally larger than his original size. In RE5 there is no less than 4 instances where someone is injected with concentrated Oroboros and within seconds has turned into a mass of flesh, muscle and tentacles, in some cases several stories tall. Without getting too science-y on this and in full knowledge that it is a video game, a transformation like that in that short a time would require more energy and fuel than a human body possesses, simply enough that kind of process would melt a person. Take note Capcom, stuff that could be real is significantly scarier than stuff everyone knows is impossible.