If you use clumping litter, how big are the pee clumps? 
My two cats who use the litter box consistently produce pee clumps that are maybe the size of a tennis ball. 
Cajun, who uses the box but who also has peed on the bed, couch and laundry pile, is producing clumps that are about the size of a golf ball.
I'm thinking there may be a urinary tract issue here, since that seems like a very small amount of pee at a time, but then it's possible that she just has a smaller bladder because she she's a very small cat. 
So do your larger cats produce more pee, and if you've had a cat with urinary issues have you noticed a corresponding change in the amount they are peeing each time? Does anyone have a healthy cat who uses the box but pees very small amounts at a time?        
        
        
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