I just watched an episode of Grimm featuring a manticore.
I'm curious do we ever see clothes torn in any episode where a wessen suddenly grows in size or grows an extra appendage like the manticore?
Recently re-watched the series, and I do not recall any Wesen losing their threads during a shapeshift. My guess is that the same "magic" that protects their true forms from humans also keeps their clothing intact. Or, possibly, their clothes are made to fit their Wesen forms (or they buy clothes in those sizes), and what we humans normally see is the overall glamor. Not sure! It would be a fun answer for the writers to give the viewers someday!