What makes a rear derailleur prone to mud/grass clogging?

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Based on a number of CX races I've done, my old-fashoined 10sp Shimano (possibly Tiagra?) derailleur seems to pick up a lot less grass+mud than other - usually more modern - bikes. (I say "usually" .. since I started racing, I am one of about 3 riders who have not yet ditched canti brakes, and gone to 1x!)

Is it just the longer cages on 1x setups which - being closer to the grass -pickup the grass more quickly? Or are some jockey wheels more vulnerable? Or some cage designs?
 
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