MrBen - most men have one testicle larger/smaller than the other - the same as most women's breasts.
It's quite common with feet too.
Having said that, anyone with any medical concerns regarding testicles or breasts should see their Doc or Practice Nurse in the first instance.
I say this, as I have had two friends who found lumps in their breasts but were too afraid to go to the Doc's -unfortunately for them & their families, they had left it too late.
My other friend had a lump checked in the very early stages & after many months of treatment (also losing her hair), she is fine & now back to work. Thank God.