I am afraid this is definitely a "All guys are Different" thing!
It depends on the guy how his scrotum and testes are, whether they are large or small or hang round his knees or are tucked up inside his body! It is also quite normal for one teste to hang higher than the other - as you masturbate they naturally ascend (get pulled into the body) to protect them, what may be happening to you is that your hand is knocking your testicle as you masturbate? Or is it just the tightness that gives you the pain?
The following information is an extract pasted from a previous answer I gave to another post which I think will help answer your question (there is a link to that post later):
"The testes are designed to move up and down, according to how warm they are so they keep the sperm at the correct temperature. But there are also other factors determining how low they hang, such as sexual excitement and if you are doing excercise. There is I believe a reflex action tht the body makes when any excercise is started to draw the testes towards the body so they are not swinging around madly where they can get hurt, and of course sex in a sort of excercise!
And of course every man will be different in how his individual testes hang. Some willl hang lower than others, some will be tight all the time, some may go back into the body completely. Ian Fleming describes in a James Bond book how this can be used by martial artists to protect the groin area!
From personal experience, mine can hang nicely in the sack in warm weather, after a warm bath etc. but in cold weather, during and after excercise (yep they look tiny in the locker room!) and of course during sex they withdraw up into the body fairly quickly only coming back out after ejaculation."
To follow on from this and answer more specifically your question, firstly there is nothing you can do about the size of your testicles. Sorry thats it! Unless you are still young enough that they still have some growing to do, of course!
But you can do things about how low they hang, and how big your scrotum is! Not much, but some! I used to have a very tight scrotum that apart from rare times in hot weather or in a hot bath would be small and tight. Some years ago I started to grab the whole sack and pull downwards, gently but firmly. Doing this several times a day (lots sometimes) I eventually ended up with a scrotum that would not just be a tight piece of skin but a sack, and as such my testes lowered into this sack nicely. Of course they still leap up into my body in cold weather or hitting cold water but the over all effect is that I have slightly lower hanging balls most of the time.
Once the scrotum is a bit looser and there is room for the testes to hang lower I did then pull on the testes a bit, to help them lower! I am not sure if this is relevent my testes did not descend until I was at least 5 or 6, so I am guessing that my scrotum didnt start life needing to be very big. They can still easily go back into my body and still do this as I masturbate, but generally they hang lower than they used to!
A quick look at the positive here - you will never need a jockstrap as these are only any use if you have low hanging testes!
I hope this helps you, sorry about the length of this post.
The full thread that I quoted fromis here: http://www.healthboards.com/boards/s...03#post2117803
, which I hope is an allowed link! If not I can always copy the info later