Hi Ed,
Yeah, I fell for the "gold" battery scam back in the iPhone 4 days. In general, Li-Ion batteries' capacities are limited by the physical size of the battery; unlike rechargeable technologies like Ni-Mh or Ni-Cad batteries, there haven't been any significant improvements in power density for Li-ion batteries; at least not enough to account for those gold battery claims.
@oldturkey03 did a great write-up on the subject that's worth a look.
[guide|95930|The myth of the "high capacity" batteries - iFixit Repair Guide|new_window=true]
So yeah, pretty much the only way to increase the capacity of a lithium ion battery is to increase the physical size. If it's the same size as another battery but claims like 50% more amperage, it's a lie.