Appropriate Guinea Pig Testicle size?

Question
My son has a 3 year old maleGuinnea Pig.  He is very good with him and takes care of him almost 100%.  While he was away at camp I cleaned his cage and while I was holding him noticed that his testicles were larger then I imagined they should be - however - I don't really know what they are supposed to look like.

Have not been able to find anything to help me guage if he is OK or needs a trip to the vet.

Any resources or pictures you could point me to as a reference?

Answer
Some male guinea pigs, especially at your guinea pig's age can have very large testicles, I have also noticed from my breeders and rescues that when males are not used as breeders they tend to have a lot bigger testicles than ones that are used as breeders. I have never had or heard of a case of testicular cancer in male guinea pigs before, but if one side is bigger than the other then that might be something to worry about, other than that I would not be too conserned unless he starts showing signs of illness. This is one of the biggest pictures I have seen of guinea pig testicles of a male, any bigger than this I might worry alittle about. (It is picture #24)
http://www.cavyspirit.com/sexing.htm
Let me know if you have any more questions.
Brittany