I'm not an expert.  My cats arrived here as adults.   Kitty FC can purr so
hard you can hear her in the next room.   Boyfriend's purr (not freely
given, btw) is almost silent.
I was in the bathroom tonight and Boyfie was sitting a few feet away.  He
feels it's essential that I need help on the toilet.  I heard purring;  I
thought it was him.  Nope, it was Kitty sitting on top of the stairs out of
sight.
Cats must vary a lot about what makes them purr.  Kitty will purr when she
is eating if she is enjoying her meal and will sometimes purr when she is
drinking if she is particularly thirsty.  She purrs if she jumps onto my
knee.
Boyfriend rarely purrs at all.  When he does I can hardly hear it.

Tweed

I would like to know the same.  I named Rumble for how loud  he was when he
was a kitten.  Now when I hear him purring it is because he is laying on my
head while I am sleeping trying to get my attention. :)

Most of the cats I got as kittens quieted down as adults - Spooky
didn't purr out loud for over ten years, but he started again when he
was about twelve.  Only a few of my adults purr loudly enough to be
heard more than a few inches away, mostly just CJ.  The kittens can be
heard across the room though.

I think the volume of the purr is probably the same - it's
just surrounded by a lot more cat than when they were
kittens!  (Flesh and fur are not very good conductors of
sound waves.)

It can also go the opposite way.  Vino's purr used to be very quiet, had
to have your ear quite close to his body to hear it.  But ever since
he's recovered from his blockage, his purr volume has increased about 10
fold.  Now he's got this super loud motor that can be heard from a few
meters away.

from what i read, kittens purr to sooth themselves or when they are
really relaxed and comfortable.  they also kneed for same reasons.
this behaviour may stop in older cats.  Shaina used to kneed a lot, but
didn't purr all that much, unless i was petting her, which has been
hard to do after she was about 2 months old because she started running
about like an electron and only set quietly when i petted and scratched
her head and neck.  she allowed less than 5 min for that activity.
since i've moved and she's not getting nearly as much attention, she
purrs much more.  whenever i pet her, pick her up, when i come back, or
feed her, she purrs a lot.  probably because she is happy to see me
(know, presumptuous human).

Both of mine purr loudly. Bagheera is just over three, people on the phone
can sometimes hear him. I jokingly say to him , "can't you be quiet? I'm
trying to listen" Snoopy purrs loudly enough to be heard from the other side
of the room.