They're not all created equal. Here are some sizes I measured with my cheap Harbor Freight digital caliper. It reads in 3 decimal points +/-.ooo5. I grabbed 3 of each at random.
Crosman Copperheads, copper coated, 6000 ct, .172 .1715 .1715
Daisy standard zinc plated, 6000 ct, .172 .1715 .1715
Avanti precision ground, .173 .173 .173
Marksman premium grade, brass plated, .1765 .1765 .177
H&N Smartshot, copper plated lead, .1715 .171 .1715
Gamo lead rounds, .177 .177 .1775
What I can deduce from this is that the Gamo and Marksman are designed for dual-ammo barrels, and may not work in BB-only barrels. The H&N Smartshot is designed as a steel-BB alternative for when you may want additional weight and/or reduced ricochets.
My Schmeisser (yes, I know I shouldn't call it that, it's an MP-40 replica, but you know what I mean) came with Hornady Black Diamond anodized BBs and I think they are impressive. I don't have any measurements to support that but they have been very accurate and reliable.
200 ml bottle shown for scale only, may cause cancer in state of California, etc.