Yup
It does have a pretty significant amount of space in the center relative the the width of the antenna windings. The antenna itself blocks most of the flux from traveling through it, so you can imagine half of the flux hitting the interior side of the antenna turns is squeezed towards the center contributing to the read, and the other half hitting the exterior side is pushed away and wasted.
It could do with a bit more space in the center for better performance, but because of manufacturing constraints with the silver foil printing process and honestly probably a lack of engineering prowess from the designer (could have put some antenna on the bottom too) they needed that large antenna on top to tune the circuit properly without the option of a tuning capacitor. Fortunately the large antenna and small input capacitance on the chip really increases the Q factor which is another thing that impacts performance.
That’s just what’s available. I would like to make it thicker, but 3oz/ft^2 copper is pretty specialized and expensive and would only bump it up to 0.1mm thickness. They antennas are pretty durable despite this. I’ve done a bunch of durability testing. No implant is perfect but I feel these are more than adequate. I’ve also learned a bunch of durability tricks over the years to prevent metal fatigue or tearing on the copper like teardropping and planning via and component placement to reinforce areas and prevent flexing.
Like Amal mentioned the, the wire antennas had issues with rubbing between the windings wearing off the enamel and causing shorts. There’s also an argument to be made that a length of copper with a rectangular cross section is more resistant to full-reversed loading fatigue than a length with a cicrular cross section. Unfortunately I don’t think I’ll be able to do the exact math to prove than hypothesis because the properties of the copper are not only different between the FPC and the wire, but the nominal properties of the annealed copper sheets used in the FPC change during the PCB fabrication process.