I believe cat358B means that a norrow bucket will have you trying to get close to work and running over things with your tires, damaging whatever you hit or possibly damaging your tires or machine. I could be totally off base too.
Keep in mind that a lighter, thinner material, cheaper bucket will flex and bow from lifting heavy loads, been there, done that. We were doing a lot of concrete demolition with it to though.
CCMoe