This has been bugging me for quite a while now, since I'm a bit of a cosmetic and outfits addict, but Wolfgang will not have the sleeves of any torso cosmetic (that doesn't belong to him), unless you equip the Torn Wolfman Trousers.
To note: This is only seen when Wolfgang is in normal or mighty form. Wimpy Wolfgang will keep the sleeves no matter what.
I have tested every other leg cosmetic and none of them have the same feature as the torn wolfman trousers. I have attached an example of a random shirt a random leg cosmetic, and then the same shirt with the Torn Wolfman Trousers.
Example: Ugly deerclops sweater with uniform pants
Ugly deerclops sweater with Torn Wolfman Trousers
1. Equip Wolfgang with any shirt cosmetic (that isn't his own)
2. Note the lack of sleeves
3. Equip Wolfgang with the "Torn Wolfman Trousers" legs cosmetic
4. Note the presence of sleeves