Unless some vendor is hiding a technical revolution (instead of patenting it), there's no way you can pack more capacity into the same volume. Be very, very sceptical of any battery with the same size as the original but claiming substantially higher capacity. Such batteries don't exist, so it's just snake oil.

And looking for smaller capacity increases in the same volume isn't much use either, as the capacity varies some +-10% anyway - what the capacity is reported as is basically decided by how conservative you are in your advertising.

If you want more battery capacity you'll have to wait for Mugen or someone to provide a physically larger battery together with a new back cover.