why is the "face of bo" just a head on "Doctor Who" on a few of the episodes..?

k, i get that he's immortal, but on an episode of doctor who, jack
harkness says how he's been stabbed and impaled and such, yet theres
no mark showing that he should've healed from it, so why is he just a
head, and even if his body was seperated from his head and was put in
a jar, why did it go so big? thats like saying that if you get a worm,
then you chop it in half, then the side thats still living should grow
to about 100x the original size..