I have experienced significant differences in salesmen, and dealers in general.
Locally there is a huge GM/Cadillac dealer... I always check in with them when I want to trade or just buy. I always get a high degree of handling by salesmen, managers, and appraisers. And then a few days pass and I hear back with an offer, usually much lower than my research tells me.
As a backup I traveled 120 miles south to another GM dealer... and I worked exclusively with one person until it was time to sign the papers. I have bought 4 vehicles from them in 7 years, and have an order in for the 5th. This salesman knows his stuff, and minimizes the BS. He makes the calls to find the vehicle I want, and horse trades with other dealers to get it.
Being bigger is not anyway linked to being better in the auto business.