I found one not-awful discussion of this on the C7 site: http://forums.cubicle7.co.uk/viewtopic.php?f=7&t=1943

It feels like they settled on “horses can carry as much treasure as needed” and also talked about the potential consequences of losing their horses. There’s one angle that says since Bilbo hauled a lifetime of treasure home on a pony, and a lifetime of treasure is 500 TPs, therefore a pony can carry 500 TPs. Not awful but it’s also such a very high number.

Also: I do this reflexive little eyeroll-giggle thing every time someone solemnly invokes The Professor when they’re talking about Tolkien. Oh heavens.