My cats' pee clumps in the box vary pretty widely.
My cats are 9.5 and 7 lbs respectively, and I find a lot of golfball-sized (or even smaller) clumps, along with some that are as large as baseballs. Considering your cat is much larger than mine, the size you describe doesn't sound abnormal to me. I don't know if soft clumps are getting stuck together or if they just pee a lot sometimes, but from what I've seen, it's normal for them to sometimes be large, and quite normal for them to pee more than once a day.
If you see him rushing to the litterbox a lot though, that's something to be concerned about. But from what it sounds like, when you're home, you don't see him go to pee that often, right? From what I understand of diabetes (I had a diabetic dwarf hamster in college) it tends to present as frequent urination (i.e. peeing very many times in a day or a few hours), rather than an abnormally large amount of pee in one trip.