The 2 pulleys use the same bearing (IIRC), but the tensioner pulley is 76mm dia. (which is pretty much exactly 3.0 inches), while the idler-pulley is noticeably larger at 91mm diameter (call it 3-5/8"...) Since both pulleys are non-driving and not running an accessory, their exact diameter is only relevant to the size of the belt needed to keep the tensioner in its Operating-Range... well, I guess you could also say using too-small of an idler-pulley means a bit less contact circumference for the 2 adjacent "accessories", the alternator and A/C compressor... (or is it the Power Steering pump?... )
... look, why anger The FoMoCo Engineering Team? They would get kudos from mgt. for using the same pulley twice (aka Reduced Bill of Materials), but decided that a larger idler-pulley was NEEDED to keep things "toit", snug, and non-squeaky! Tell those parts store loo-HOO-zers to give you the "big" pulley... "size matters"...