Why is a phone's NFC antenna so small?
Because everything inside a phone competes for the same space and NFC is not the priority. That single constraint sets the ceiling on read distance for every tag you will ever deploy, which is why the reader rather than the tag is the limiting half of the system.
What it competes with
A camera assembly that grows every generation. A battery filling as much of the volume as possible. A wireless charging coil in the middle of the back. Structural metal holding the whole thing together.
NFC gets the space left over, and it has to be space with no metal behind it.
It also has to work through whatever case the owner adds, which is a constraint the designer does not control.
It sets the ceiling
Coupling depends on how much of the reader's field passes through the tag's coil, and the reader's field is concentrated around its own coil.
A small reader coil produces a field concentrated in a small region that falls away quickly. That is the ceiling, and no tag can raise it.
This is why a card sized tag and a much larger tag read at similar distances from a phone. Past a point the tag's extra area sits outside the field entirely.
And it is why a dedicated reader with a large antenna changes the answer completely: it lifts the ceiling rather than working under it.
The consequence for tags
Range with a phone is a contact range technology in practice, whatever figures are quoted for the technology as a whole.
Buying a bigger tag helps up to about card size and then stops helping, which is the opposite of what unit pricing encourages.
Effort spent on placement, surface and a visible target returns more than effort spent on tag size, once you are past a small sticker.
None of this is a fault in your tags. It is the shape of the reader you are designing for.
Designing with the limit
Design for a touch rather than a wave, and say so where members of the public are involved.
Use a card sized tag where the object allows it, and stop there.
Put the tag where a phone naturally arrives, which matters more than the last centimetre of theoretical range.
And where the requirement genuinely exceeds contact, change the reader. That is a different project and saying so early is cheaper than discovering it after installation.
Frequently asked questions
Could a phone have a bigger NFC antenna?
Physically yes, at the cost of something else inside. It has not been a priority, which is why the constraint has held across generations.
Does a bigger phone have a bigger coil?
Not meaningfully. A larger back gives more surface and the coil stays small, so the usable region is a smaller fraction of it.
Why do quoted NFC ranges sound larger?
Because they describe the technology with a generous reader. A phone sits at the bottom of that range.
Would a better tag help?
Up to about card size, yes. Beyond that the extra area sits outside the field a phone produces and contributes nothing.