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        Hi Myth, 
I'm not sure, but it sounds like a territorial problem Cats are individuals, territorial animals. You say one is bigger, the other smaller, by this I take that you mean older and younger - am I right? And there's another question -are they both castrated? 
What I think is that the younger one is not comfortable to use a litter box "marked" already by the other's scent, as his territory. 
We, humans, are poorly equipped when it comes to smell - what we find completely odorless, might be a scent "treasure box" for them, by which I mean they can also find it really "stinking" - if they don't enjoy what they smell. And I repeat: for us it will probably completely odorless! 
Males mark their territories spraying objects with a liquid produced in glands attached to their testis - these gland are removed when they are castrated, so that they won't go on "marking" your walls, doors, etc. 
If you can, I would suggest for you to give the other cat a private litter box,and make it accessible only to the young one, for a while, until it's marked with his scent. If he's already castrated, don't worry, his urine scent will do the job.  
I hope this will help. 
Please let me know if there's any improvement. 
And give them a "chin rub" on my behalf! 
Millie:) 
p.s. I assume the cat is not vindictive, just feels he has no chioce. 
m.
     
     
    
    
    
    
    
    
  
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