In the description of the transmute rock to mud spell is a clause that says the mud is 10-feet deep, but creatures only sink hip- or chest-deep in the mud. I need to understand this statement, since it sounds to me like a Medium-size creature sinks further than a Small-size creature. Why? Other descriptions, for example quicksand (link to the RSRD), require the creature to roll Swim checks or drown.

Any comments as to why the spell is written that way? Would I be remiss if I ruled that the creatures sink to the bottom of the mud?