shoulders look "thin" when in a position other than the t-pose

if you make the neck shorter then the arms will look longer

it looks thin because C4D is reading the weight map wrong... in fact its not reading it at all and is just giving a standard "one size fits all" map which is a common mistake that Blender also makes

what to do about it = fit it in C4d {assuming that old software can even make weight maps} or use something more up to date like Iclone